Remove 2>&1 from nix build capture. When repo is dirty, nix emits
warnings to stderr which were being merged into $out and used as
symlink targets, creating broken symlinks like:
orch -> warning: Git tree '...' is dirty\n/nix/store/...
Now stderr goes to terminal, only stdout (store path) captured.
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
