flake8: run the workflow conditionally

We don't need to run flake8 on ansible modules and their tests if we
don't have any modifitions.

Signed-off-by: Dimitri Savineau <dsavinea@redhat.com>

.github/workflows/flake8.yml

@@ -1,5 +1,11 @@
 name: flake8
-on: [pull_request]
+on:
+  pull_request:
+    paths:
+      - 'library/**.py'
+      - 'tests/conftest.py'
+      - 'tests/library/**.py'
+      - 'tests/functional/tests/**.py'
 jobs:
   build:
     runs-on: ubuntu-latest