ubuntu-slim runners (Hosted Compute Agent Docker containers) are
GitHub-hosted but lack the standard USER environment variable set
on full VM-based runners. This causes chownForFolder to fail with
'chown: invalid user: undefined'.
Instead of patching chownForFolder, detect ubuntu-slim early
informative message, matching the existing patterns for isDocker(),
isARCRunner(), and other unsupported runner types.
Fixes#627
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
376d25a97f