Bug 1754241: ./mach lint now raises exception on no-op in CI r=ahal

If there's no files to lint in CI, that's a smell that something is
misconfigured.

Resolve this by raising an exception accordingly if `MOZ_AUTOMATION` is
set (and we aren't running the tests for the linting code itself).
Automatically recommend specifying `*`, as (assuming the use of a
shell) that will resolve to "all files/dirs at $topsrcdir`", which
matches existing behaviour if `--outgoing` and `--workdir` aren't
provided.

Differential Revision: https://phabricator.services.mozilla.com/D138172

python/mozlint/mozlint/roller.py

@@ -337,6 +337,14 @@ class LintRoller(object):
                 print(e.output)
 
         if not (paths or vcs_paths) and (workdir or outgoing):
+            if os.environ.get("MOZ_AUTOMATION") and not os.environ.get(
+                "PYTEST_CURRENT_TEST"
+            ):
+                raise Exception(
+                    "Despite being a CI lint job, no files were linted. Is the task "
+                    "missing explicit paths?"
+                )
+
             print("warning: no files linted")
             return self.result