AppVeyor: Build on all PRs

We should look into changing [this setting](https://github.com/cefsharp/cef-binary/blob/master/appveyor.yml#L9), 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](https://github.com/cefsharp/cef-binary/blob/master/build.ps1#L49) to set the NuGet package version [like this]($env:APPVEYOR_BUILD_VERSION), unless `$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.

Start with the AppVeyor configuration at appveyor.yml line 9 and the package-version logic in build.ps1 around line 49. Check how pull-request builds currently run and how tagged builds set NuGet versions. Done means all PRs build, CI builds use the AppVeyor build version, and tagged builds retain the repository tag version.

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