Make CoreCLR and Libraries live asset OverridePath/ArtifactPath normalization more consistent

[dagood]

These lines take override paths and normalize them before using as artifact dirs:

runtime/eng/liveBuilds.targets

Lines 12 to 16 in a773bc3

	<!-- Accept override paths for live artifacts. -->
	<PropertyGroup>
	<CoreCLRArtifactsPath Condition="'$(CoreCLROverridePath)' != ''">$([MSBuild]::NormalizeDirectory('$(CoreCLROverridePath)'))</CoreCLRArtifactsPath>
	<LibrariesArtifactsPath Condition="'$(LibrariesOverridePath)' != ''">$([MSBuild]::NormalizeDirectory('$(LibrariesOverridePath)'))</LibrariesArtifactsPath>
	</PropertyGroup>

We should consider also doing this for LibrariesAllConfigurationsOverridePath.

It is necessary for LibrariesOverridePath and CoreCLRArtifactsPath, but LibrariesAllConfigurationsOverridePath is not necessary. However, having them all follow the same pattern makes the code more predictable/readable.