[nrf fromtree] ci: coding_guidelines: Use Ubuntu 20.04 runner image

This commit updates the coding guidelines workflow to use a specific
runner image version, ubuntu-20.04, instead of the latest version in
order to prevent any potential breakages due to the 'latest' version
change by GitHub.

Signed-off-by: Stephanos Ioannidis <[email protected]>
(cherry picked from commit 1a4105b6b526db5e8dcab59492453cc7f858fb9f)
Signed-off-by: Thomas Stilwell <[email protected]>

Author: stephanosio

.github/workflows/coding_guidelines.yml

@@ -4,7 +4,7 @@ on: pull_request
 
 jobs:
   compliance_job:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     name: Run coding guidelines checks on patch series (PR)
     steps:
     - name: Checkout the code