Outer build of multitargeted projects doesn't respect WarningsNotAsErrors

[rainersigwald]

The promotion of the "plain property" version of some warning-related settings:

msbuild/src/Tasks/Microsoft.Common.CurrentVersion.targets

Lines 668 to 672 in 69b3e7a

	<PropertyGroup>
	<MSBuildWarningsAsMessages Condition="'$(MSBuildWarningsAsMessages)'==''">$(NoWarn)</MSBuildWarningsAsMessages>
	<MSBuildWarningsAsErrors Condition="'$(MSBuildWarningsAsErrors)'==''">$(WarningsAsErrors)</MSBuildWarningsAsErrors>
	<MSBuildWarningsNotAsErrors Condition="'$(MSBuildWarningsNotAsErrors)'==''">$(WarningsNotAsErrors)</MSBuildWarningsNotAsErrors>
	</PropertyGroup>

Is in Microsoft.Common.CurrentVersion.targets, which is not imported into multitargeting outer builds--only the inner builds. This means that any warnings raised in the outer build (like in NuGet operations) may be promoted to error even if $(WarningsNotAsErrors) includes them.

This is very related to #10877 and we should consider fixing both by looking at the unprefixed versions in the engine (if the prefixed version is unset) rather than requiring XML logic to do it.