Revert "Mount plugin folders on behalf of the user owning the wsl session" (#41331) (#41515)

Temporarily reverting the identity-based plugin folder mount change
(both the WslCoreVm.cpp behavior change and the accompanying
MountFolderAccess test coverage) introduced in #41331.

This reverts commit 78b9cf201df66ea470dc6de096ef6722905ba785.

Co-authored-by: Ben Hillis <benhill@ntdev.microsoft.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>