fix: ensure lockfiles are files

The change ensures that lockfiles that are not files are filtered out before processing. This change aligns with the corresponding one made in the CLI (phylum-dev/cli#1202). It is done separately here because #244 (Replace lockfile detection with `phylum status`) has not been implemented yet. Plus, this allows for a fix without changing the minimum supported CLI version.

src/phylum/ci/ci_base.py

@@ -145,6 +145,10 @@ def filter_lockfiles(self, provided_lockfiles: List[Path]) -> Lockfiles:
                 LOG.warning("Provided lockfile does not exist: %s", provided_lockfile)
                 self.returncode = ReturnCode.LOCKFILE_FILTER
                 continue
+            if not provided_lockfile.is_file():
+                LOG.warning("Provided lockfile is not a file: %s", provided_lockfile)
+                self.returncode = ReturnCode.LOCKFILE_FILTER
+                continue
             if not provided_lockfile.stat().st_size:
                 LOG.warning("Provided lockfile is an empty file: %s", provided_lockfile)
                 self.returncode = ReturnCode.LOCKFILE_FILTER