v6.24.1

Changed

• Stage 01's matrix is a script. The ~300-line inline bash of
  unity-pipeline.yml's "Resolve build matrix" step is
  scripts/common/resolve_build_matrix.sh, unchanged except that it finds
  its matrix_runner_labels.py helper beside itself instead of probing five
  candidate paths. The step's env is its whole input, as before. The test
  harness runs the file rather than a bash -c of the extracted step text.
  unity-pipeline.yml is down to about 1900 lines.