AppVeyor: Build on all PRs

[perlun]

We should look into changing this setting, so that we build on all PRs. This helps us verify changes in build.ps1 more easily.

The easiest way to fix this from what I can tell would be to tweak this part to set the NuGet package version like this, unless $env:APPVEYOR_REPO_TAG -eq "True"

(In other words, we will get junk version numbers like 62.0.0-CI34 until we tag the repo; for these builds, we will set the NuGet package version to match the tag just like we've done historically.)

Anyone wants to fix this - feel free, it's up for grabs.

[amaitland]

I've successfully pushed packages to the cef-binary project feed hosted by appveyor, see commit https://github.com/cefsharp/cef-binary/commit/76e952953d6ee6f3f441e32ee4841b8d19ec1084

The publicly accessible feed URL is https://ci.appveyor.com/nuget/cef-binary-ci

I'm only pushing packages to the feed when a tag is detected, so this should bring this one step closer. The build script can probably be changed to build a specific version if a tag is detected, with the default being to grab the most recent version available from Spotify. Thoughts?