Evaluate making the _RunRidSpecificBuild target run the rid-specific builds in parallel

[rolfbjarne]

Evaluate making the _RunRidSpecificBuild target run the rid-specific builds in parallel

The inner MSBuild task in the _RunRidSpecificBuild for reference:

macios/dotnet/targets/Xamarin.Shared.Sdk.targets

Lines 393 to 407 in bf8f472

	<!-- Execute the inner builds -->
	<!-- We force AppendRuntimeIdentifierToOutputPath=true because otherwise the inner builds might end up writing to the same directory -->
	<MSBuild
	Projects="$(MSBuildProjectFile)"
	Targets="_BuildRidSpecificAppBundle"
	Properties="
	AppendRuntimeIdentifierToOutputPath=true;
	RuntimeIdentifier=%(_RuntimeIdentifiersAsItems.Identity);
	_CodesignItemsPath=%(_RuntimeIdentifiersAsItems.RidSpecificCodesignItemsPath);
	_MtouchSymbolsList=%(_RuntimeIdentifiersAsItems.RidSpecificSymbolsListPath);
	_UserFrameworksWithoutDebugSymbolsPath=%(_RuntimeIdentifiersAsItems.RidSpecificUserFrameworksWithoutDebugSymbolsPath);
	$(_RidSpecificProperties);
	">
	<Output TaskParameter="TargetOutputs" ItemName="_AssemblyPublishDirectories" />
	</MSBuild>

We could make this task run the inner MSBuild tasks in parallel.

Potential pros:

• Faster build due to parallelization.
• Find bugs due to logic errors / double writes, etc.

Potential cons:

• If it's not safe to run rid-specific builds in parallel, any bugs are likely to be random and hard to diagnose.
• Slower build due to CPU/memory thrashing (we already AOT-compilation for instance to max out the CPU, doing it twice will make the build slower).

Things to consider:

• Only release builds are multi-rid, and we don't care as much about speed for release builds as we do for debug builds.

[baronfel]

When investigating this, it would be prudent to

• use the existing MSBuild default property BuildInParallel as a 'fallback' for this
• but also have an area-specific property so that if there are issues users can override this parallelism without tanking the rest of their builds

e.g.

<PropertyGroup>
  <MacOsInnerBuildInParallel Condition="'$(MacOsInnerBuildInParallel)' == ''">$(BuildInParallel)</MacOsInnerBuildInParallel>
</PropertyGroup>

<MSBuild
    Projects="..."
    BuildInParallel="$(MacOsInnerBuildInParallel)"
    ... />