Add support for source-build

[natemcmaster]

Resolves dotnet/aspnetcore#7282

Changes:

• ⚠️ Update build.sh/cmd to follow the same pattern used of other aspnet repos. To run tests, use build.cmd -test. We don't run tests in source-build, so this needs to be disabled.
• Add a test PR check that attempts to imitate the source-build environment
• Exclude the following from building during source-build
  • everything under src/Razor/test/
  • VSIX projects
  • MPack projects
  • non-shipping, internal tools like dotnet-razorpagegenerator
• Minor cleanup - use rolling builds to batch jobs in azure-pipelines.yml

cc @dseefeld @crummel

[dougbu]

I'm mainly questioning the need for the ⚠️ change

[dougbu]

This is a test project. Why is it restoring or building at all when '$(DotNetBuildFromSource)' == 'true'?

[natemcmaster]

I don't really understand why this project exists at all either when the projects could just be restored normally. It causes a double restore. @pranavkm

This change is necessary because the "Build" target still runs on all projects in a source build, but with dummy targets that are overrriden to no-op if ExcludeFromSourceBuild is true. See https://github.com/dotnet/arcade/blob/master/src/Microsoft.DotNet.Arcade.Sdk/tools/Empty.targets. Because this project file is using "BeforeTargets" to chain in, it too needs the exclusion to avoid restoring projects that are supposed to be ignored in a source build.

[pranavkm]

when the projects could just be restored normally

Just a regular project reference?

[natemcmaster]

Or added to the .sln file.

[dougbu]

Seems fine, especially because it's building 😺 My un-resolved suggestions stand (but they're suggestions).