[6.0] create file to trigger user local workload installation

[tmds]

For source-build to install workloads into a user folder we need a file in the installation:

The file should be created under the dotnet root at /metadata/workloads/<sdkfeatureband>/userlocal. The version does not include the -suffix. So at 6.0.100 (not 6.0.100-rc1).

@dseefeld suggested:

Can the file be created when the sdk layout is created? The way to get it only for source-built versions is to condition it on '$(DotNetBuildFromSource)' == 'true'.

I'm creating this ticket so this has some more visibility than the discussion on the closed PR dotnet/sdk#18823.

cc @dsplaisted @dseefeld @dleeapho @omajid @sfoslund @marcpopMSFT

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.