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.