github: compliance: fix python cache keys

The compliance and coding guidelines workflows are using the same pip
cache key, but they install a different set of packages. This is
probably a copy paste error from the documentation build workflow, which
has been changed to use a hash, and is causing one of the two steps to
restore from a incorrect cache.

Fix this by using a hash of the workflow file itself as a key, as that's
where the python package list is defined.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>