Add workaround for gzipped gpg keys to fix random CI failures

[fwcd]

This combines the fixes by @pfusik and @redsun82, updating the gpg URL from https://swift.org to the canonical location under https://www.swift.org and unzipping the fetched keys to work around #591. This logic is scoped to macOS and Linux for now, since these Unix commands may be unavailable on Windows.

While this is not the most elegant solution, having it upstream would fix the majority of random CI failures for Swift packages for now without vendoring a custom version of this action, until the likely server-sided misconfiguration is fixed.