casserver.py: Fix write operations with bst-artifact-server

Allow gRPC communication in the bst-artifact-server main process.

This fixes the following assertion failure in CASCache._get_local_cas() in bst-artifact-server:

assert self._fork_disabled or not utils._is_main_process()

This wasn't caught by CI as in tests the server is spawned as a subprocess.

approved this merge request

assigned to @marge-bot123

merged

mentioned in commit fcadd6b5