Fixes this random failure:
error: hash mismatch in fixed-output derivation '/tmp/nix-shell.EUgAVU/nix-test/tests/check/store/sfps3l3c5n7dabpx34kigxnfhmrwk2h6-dummy.drv':
specified: sha256-47DEQpj8HBSa+/TImW+5JCeuQeRkm5NMpJWZG3hSuFU=
got: sha256-0qhPS4tlCTfsj3PNi+LHSt1akRumTfJ0WO2CKdqASiY=
which happens because multiple tests were writing to ./dummy.
With --check and the --keep-failed (-K) flag, the temporary directory
was being retained regardless of whether the build was successful and
reproducible. This removes the temporary directory, as expected, on
a reproducible check build.
Added tests to verify that temporary build directories are not
retained unnecessarily, particularly when using --check with
--keep-failed.
