introspection failures in main

[rolfbjarne]

These introspection failures have showed up in main in different and unrelated PRs (but not in all PRs to main, nor can I reproduce locally):

[18:52:15.1236080] ?unknown-entry? !missing-null-allowed! 'Foundation.NSUrl Foundation.NSUrl::get_AbsoluteUrl()' is missing an [NullAllowed] on return type in '/Users/builder/azdo/_work/1/s/xamarin-macios/tests/xtro-sharpie/common-Foundation.ignore'
[18:52:15.1237580] ?unknown-entry? !missing-null-allowed! 'Foundation.NSUrl Foundation.NSUrl::get_BaseUrl()' is missing an [NullAllowed] on return type in '/Users/builder/azdo/_work/1/s/xamarin-macios/tests/xtro-sharpie/common-Foundation.ignore'
[18:52:15.1237810] ?unknown-entry? !missing-null-allowed! 'Foundation.NSUrl Foundation.NSUrl::get_FilePathUrl()' is missing an [NullAllowed] on return type in '/Users/builder/azdo/_work/1/s/xamarin-macios/tests/xtro-sharpie/common-Foundation.ignore'
[18:52:15.1238000] ?unknown-entry? !missing-null-allowed! 'Foundation.NSUrl Foundation.NSUrl::get_FileReferenceUrl()' is missing an [NullAllowed] on return type in '/Users/builder/azdo/_work/1/s/xamarin-macios/tests/xtro-sharpie/common-Foundation.ignore'
[18:52:15.1238140] ?unknown-entry? !missing-null-allowed! 'Foundation.NSUrl Foundation.NSUrl::get_StandardizedUrl()' is missing an [NullAllowed] on return type in '/Users/builder/azdo/_work/1/s/xamarin-macios/tests/xtro-sharpie/common-Foundation.ignore'
[18:52:15.1246020] ?unknown-entry? !missing-null-allowed! 'System.String Foundation.NSUrl::get_AbsoluteString()' is missing an [NullAllowed] on return type in '/Users/builder/azdo/_work/1/s/xamarin-macios/tests/xtro-sharpie/common-Foundation.ignore'
[18:52:15.1246550] ?unknown-entry? !missing-null-allowed! 'System.String Foundation.NSUrl::get_Fragment()' is missing an [NullAllowed] on return type in '/Users/builder/azdo/_work/1/s/xamarin-macios/tests/xtro-sharpie/common-Foundation.ignore'
[18:52:15.1247070] ?unknown-entry? !missing-null-allowed! 'System.String Foundation.NSUrl::get_Host()' is missing an [NullAllowed] on return type in '/Users/builder/azdo/_work/1/s/xamarin-macios/tests/xtro-sharpie/common-Foundation.ignore'
[18:52:15.1248040] ?unknown-entry? !missing-null-allowed! 'System.String Foundation.NSUrl::get_LastPathComponent()' is missing an [NullAllowed] on return type in '/Users/builder/azdo/_work/1/s/xamarin-macios/tests/xtro-sharpie/common-Foundation.ignore'
[18:52:15.1248260] ?unknown-entry? !missing-null-allowed! 'System.String Foundation.NSUrl::get_Password()' is missing an [NullAllowed] on return type in '/Users/builder/azdo/_work/1/s/xamarin-macios/tests/xtro-sharpie/common-Foundation.ignore'
[18:52:15.1248560] ?unknown-entry? !missing-null-allowed! 'System.String Foundation.NSUrl::get_Path()' is missing an [NullAllowed] on return type in '/Users/builder/azdo/_work/1/s/xamarin-macios/tests/xtro-sharpie/common-Foundation.ignore'
[18:52:15.1249100] ?unknown-entry? !missing-null-allowed! 'System.String Foundation.NSUrl::get_PathExtension()' is missing an [NullAllowed] on return type in '/Users/builder/azdo/_work/1/s/xamarin-macios/tests/xtro-sharpie/common-Foundation.ignore'
[18:52:15.1250050] ?unknown-entry? !missing-null-allowed! 'System.String Foundation.NSUrl::get_Query()' is missing an [NullAllowed] on return type in '/Users/builder/azdo/_work/1/s/xamarin-macios/tests/xtro-sharpie/common-Foundation.ignore'
[18:52:15.1250530] ?unknown-entry? !missing-null-allowed! 'System.String Foundation.NSUrl::get_RelativePath()' is missing an [NullAllowed] on return type in '/Users/builder/azdo/_work/1/s/xamarin-macios/tests/xtro-sharpie/common-Foundation.ignore'
[18:52:15.1251060] ?unknown-entry? !missing-null-allowed! 'System.String Foundation.NSUrl::get_ResourceSpecifier()' is missing an [NullAllowed] on return type in '/Users/builder/azdo/_work/1/s/xamarin-macios/tests/xtro-sharpie/common-Foundation.ignore'
[18:52:15.1251690] ?unknown-entry? !missing-null-allowed! 'System.String Foundation.NSUrl::get_Scheme()' is missing an [NullAllowed] on return type in '/Users/builder/azdo/_work/1/s/xamarin-macios/tests/xtro-sharpie/common-Foundation.ignore'
[18:52:15.1252330] ?unknown-entry? !missing-null-allowed! 'System.String Foundation.NSUrl::get_User()' is missing an [NullAllowed] on return type in '/Users/builder/azdo/_work/1/s/xamarin-macios/tests/xtro-sharpie/common-Foundation.ignore'
[18:52:15.1255990] ?unknown-entry? !missing-null-allowed! 'System.String[] Foundation.NSUrl::get_PathComponents()' is missing an [NullAllowed] on return type in '/Users/builder/azdo/_work/1/s/xamarin-macios/tests/xtro-sharpie/common-Foundation.ignore'
[18:52:15.3414530] ?unknown-entry? !missing-null-allowed! 'Foundation.NSUrl Foundation.NSUrl::FromPasteboard(AppKit.NSPasteboard)' is missing an [NullAllowed] on return type in 'macOS-AppKit.ignore'

https://github.com/xamarin/xamarin-macios/pull/15439#issuecomment-1179843954

[mandel-macaque]

This was because the bots had all pkgs from previous runs in the download folder. The code was installing the first pkg it found. Once I fixed that, the random failures went away.