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

This commit updates the compliance check 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 f4447fc)
Signed-off-by: Thomas Stilwell <[email protected]>

Author: stephanosio

.github/workflows/compliance.yml

@@ -4,7 +4,7 @@ on: pull_request
 
 jobs:
   maintainer_check:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     name: Check MAINTAINERS file
     steps:
     - name: Checkout the code
@@ -22,7 +22,7 @@ jobs:
         python3 ./scripts/get_maintainer.py path CMakeLists.txt
 
   check_compliance:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     name: Run compliance checks on patch series (PR)
     steps:
     - name: Update PATH for west