Don't run mypy_primer on stubsabot PRs (#9332)

We can be fairly confident that PRs updating only a `METADATA.toml` file will never produce an interesting `mypy_primer` diff.

.github/workflows/mypy_primer.yml

@@ -5,7 +5,7 @@ on:
   pull_request:
     paths:
     - 'stdlib/**'
-    - 'stubs/**'
+    - 'stubs/**/*.pyi'
     - '.github/workflows/mypy_primer.yml'
     - '.github/workflows/mypy_primer_comment.yml'