Reconcile OSName in source-build with repo usage

[dseefeld]

An issue with 5.0 building on OSX turned out to be that we were setting OSName incorrectly in the installer build. I think the issue may exist in 3.1 as well. OSName is checked in some of the repos as "$(OSName)" == "osx" but we're setting it to something like osx.10.14 in this code:

source-build/repos/cli.proj

Lines 5 to 7 in 05e2a03

	<OverrideTargetRid>$(TargetRid)</OverrideTargetRid>
	<OverrideTargetRid Condition="'$(TargetOS)' == 'OSX'">osx-x64</OverrideTargetRid>
	<OSNameOverride>$(TargetRid.Substring(0, $(TargetRid.IndexOf("-"))))</OSNameOverride>

That same code was in installer.proj in 5.0 (copied over). This was causing the check to fail in the installer repo and it was setting CreateCrossgenSymbols to true here: https://github.com/dotnet/installer/blob/efcb9c319c41105e769afcafbd6c8860f7411f25/src/redist/targets/Crossgen.targets#L107-L109

The fix in 5.0 was to update this line:

source-build/repos/cli.proj

Line 7 in 05e2a03

<OSNameOverride>$(TargetRid.Substring(0, $(TargetRid.IndexOf("-"))))</OSNameOverride>

to the following:

<OSNameOverride>$(OverrideTargetRid.Substring(0, $(OverrideTargetRid.IndexOf("-"))))</OSNameOverride>

[MichaelSimons]

[Triage] This code has been refactored some since it was originally opened. The OSX issue is N/A until #2909 is implemented in .NET 8.