The check's results are already embedded in its builder text — the comment
above `runCommand` says so — but nothing said that this makes the verdict
readable straight out of the derivation. So mutation-testing a case, which
is the documented way to prove a case can fail, has been costing a build on
the shared farm per arm when it costs none.
Command verified against both arms: on an unmutated tree it prints the
`N properties hold` line, and with a case deliberately broken it prints that
case's `FAILED:` line. The jq shape is envelope-agnostic, since `derivation
show` grew a `{"derivations": …}` wrapper.
c90b999285