Broke ability to use RuntimeFlavor in local builds

[jaredpar]

Execute the following command:

.\build.cmd /p:RuntimeFlavor=Mono

Will lead to the following error:

P:\runtime\eng\liveBuilds.targets(91,5): error : The CoreCLR artifacts path does not exist 'P:\runtime\artifacts\bin\coreclr\Windows_NT.x64.netcoreapp5.0-Windows_NT-Debug'. The CoreCLR subset category must be built before building this project. [P:\runtime\src\libraries\System.Reflection.Primitives\src\System.Reflection.Primitives.csproj]

There are two issues at hand here that need to be addressed:

1. The runtime configuration logic in our build files is not properly accounting for times when $(Configuration) is set but $(ConfigurationGroup) is not.
2. There are still places in the build logic that do not properly condition grabbing runtime files based on the chosen runtime

This was a regression introduced with #2093. That means we have the following chain of sadness: PR #1934 leads to runtime config breaks leads to #2093 which leads to this.

[akoeplinger]

Is there still something to be done after #24077? seems to be working here.

[jaredpar]

@akoeplinger

I was keeping it open for the second part of the issue

There are still places in the build logic that do not properly condition grabbing runtime files based on the chosen runtime

That's a separate issue. Can track separately.