Reenable parallel msbuild execution - 2x faster test build

[4creators]

Fixes #19386 regression introduced in #19324

cc @AaronRobinsonMSFT @BruceForstall

[BruceForstall]

@dotnet-bot test Windows_NT x64 Build and Test

[BruceForstall]

@dotnet-bot test Windows_NT x86 Checked Build and Test

[AaronRobinsonMSFT]

@dotnet-bot tests Ubuntu x64 Checked Innerloop Build and Test

[BruceForstall]

Look like random/expected test failures, but try again:

test Ubuntu x64 Checked Innerloop Build and Test
test Windows_NT x86 Checked Build and Test

[4creators]

All Windows failing tests are timing out

[4creators]

https://ci.dot.net/job/dotnet_coreclr/job/master/job/x86_checked_windows_nt_prtest/14697/

Test Result (4 failures/+4)
baseservices_threading._generics_threadstart_GThread24_GThread24_._generics_threadstart_GThread24_GThread24_cmd
baseservices_threading._generics_threadstart_GThread22_GThread22_._generics_threadstart_GThread22_GThread22_cmd
JIT_Generics._Coverage_chaos65204782cs_o_chaos65204782cs_o_._Coverage_chaos65204782cs_o_chaos65204782cs_o_cmd
Regressions_coreclr._1514_InterlockExchange_InterlockExchange_._1514_InterlockExchange_InterlockExchange_cmd
--

[4creators]

@dotnet-bot test OSX10.12 x64 Debug CoreFX Tests

[4creators]

@dotnet-bot test OSX10.12 x64 Debug CoreFX Tests

[4creators]

Previous OSX failure was a timeout after more than 7 hour run

https://ci.dot.net/job/dotnet_coreclr/job/master/job/x64_debug_osx10.12_corefx_innerloop_prtest/34/

[4creators]

Related issue #19403

[4creators]

OSX filed again due to timeout:

11:01:29 Build timed out (after 420 minutes). Marking the build as aborted.
11:01:29 Build was aborted

[4creators]

@BruceForstall IMO we can merge it since failures are unrelated

[BruceForstall]

@4creators Thanks for keeping an eye on this; it's great to have this improvement.