Revert a revert of a merge that shouldn't have been in master but was intentionally in staging.
Next time I'll do this right after the revert instead of so far down the line...
This reverts commit 9adad8612b.
Newer 'mutagen' versions have a test suite that trips over missing
encoding in this file. The file is copied to source trees at build time
with the name nix_run_setup.py.