Embed executables in API JSON

- Avoid fetching `executables.txt` as a separate API artefact.
- Populate generated JSON from GitHub Packages metadata.
- Keep `which-formula` data generated from active formula JSON.
- Fall back to GitHub Packages data during the JSON transition.
- Drop obsolete `--skip-update` command options.
- Keep stale `HOMEBREW_BOOTSNAP_GEM_PATH` from breaking coverage runs.
- Use the correct Linux mtime check for the executables database.