Skip worktree dependency symlink setup in `setup_worktree_build.py` when `build` already exists as a real directory (not a symlink), which indicates a pre-populated checkout such as a btrfs CoW snapshot rather than a normal git worktree. TAG=agy CONV=ac1407cc-76ee-4bd6-888d-4b747c3582ce Change-Id: I563ca80e9776d1dd4e1261d44cdefe82edb4cba8 Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/8615041 Reviewed-by: Olivier Flückiger <olivf@chromium.org> Commit-Queue: Jakob Kummerow <jkummerow@chromium.org> Auto-Submit: Jakob Kummerow <jkummerow@chromium.org> Commit-Queue: Olivier Flückiger <olivf@chromium.org> Cr-Commit-Position: refs/heads/main@{#111009}