[One .NET] fix parallel .sln builds

[jonathanpeppers]

Fixes: #5280

A test has been randomly failing with:

(CoreCompile target) ->
CSC : error CS2012: Cannot open 'C:\a\1\s\bin\TestRelease\temp\SolutionBuildSeveralProjects\Lib6\obj\Debug\Lib6.dll' for writing -- 'The process cannot access the file 'C:\a\1\s\bin\TestRelease\temp\SolutionBuildSeveralProjects\Lib6\obj\Debug\Lib6.dll' because it is being used by another process.'

I could reproduce this by putting [Repeat(10)] on the test. We
shouldn't commit this change, since the test took 5+ minutes.

The problem happens when:

• Build a .sln with two Android head projects that reference library
  projects.
• Use -m:4 to build with multiple MSBuild nodes.
• One of the head projects is running <Csc/> of a referenced project
  at the same time another.

The two <Csc/> calls can only occur because of the way we have inner
builds setup for $(RuntimeIdentifier) in 51fb93e. One project is
building the referenced library normally, while another is doing an
inner build for a $(RuntimeIdentifier).

We don't really want <Csc/> to compile things at all in an inner
build. It should just be "finding" paths to assemblies for each
$(RuntimeIdentifier) and not doing actual work.

Let's set $(SkipCompilerExecution) to true for the inner build:

• https://github.com/dotnet/roslyn/blob/4929b2e60abbdbb6cbe72c856d6f370c8cf13b35/src/Compilers/Core/MSBuildTask/Microsoft.CSharp.Core.targets#L132
• https://github.com/dotnet/project-system/blob/e217c5fc9f5ec06f593f74762a3bf4d750475a67/docs/well-known-project-properties.md#skipcompilerexecution-bool

This property is set during design-time builds in Visual Studio, so it
should be something we can rely on here.

I think this solved the problem, as I was unable to reproduce it
anymore after a few tries. It's possible it didn't, though!

[jonpryor]

Is this failure in CheckMultipleLibraryProjectReferenceAlias(True) expected/ignorable?

https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=4311923&view=ms.vss-test-web.build-test-results-tab&runId=17738128&resultId=100129&paneView=debug

Library should have built.
Expected: True
But was:  False


   at Xamarin.Android.Build.Tests.AndroidUpdateResourcesTest.CheckMultipleLibraryProjectReferenceAlias(Boolean withGlobal) in /Users/builder/azdo/_work/1/s/xamarin-android/src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/AndroidUpdateResourcesTest.cs:line 44

[radekdoulik]

The CheckMultipleLibraryProjectReferenceAlias test failed few times in the past on master and succeeded in re-run, so it might be unrelated.

I have restarted the remaining failing build. Otherwise the change LGTM.