Improvement:
Checks calling patchelf and grep are executed in parallel with $NIX_BUILD_CORES
Tradeoff:
One more sub-shell is spawned for each file, only if that file is a script or an ELF file.
This ensures that RPATH entries like "/foo/build/bar" doesn't trigger a
match when TMPDIR is "/build/bar". (I've had this problem with a
prebuilt package.)
On Linux the `$TMPDIR` is `/build`. The TMPDIR audit looks for `$TMPDIR`
in the build output, which will then fail with packages like
/buildkite-agent.
This fixes the heuristic to look for `$TMPDIR/` instead.
