Quiet git rm/clean output when clearing worktree on retry

When GET_SOURCES_ATTEMPTS is set and a source fetch fails, the worktree-clearing step ran git rm and git clean without a quiet flag, printing one log line per tracked/untracked file. In large repositories this floods the job log past the 4 MiB limit, truncating any output that follows, and the volume of trace writes has also been observed to slow the cleanup step significantly.

Add --quiet/-q to both commands in the shell-script generation path (shells/abstract.go) and the native runner path (functions/concrete/run/stages/get_sources.go). Git errors still surface on stderr, so failures remain visible.

Closes #39593 (closed)