Use Ubuntu-20.04 runner for running header guard validation

Using `ubuntu-latest` is triggering warning that it will soon migrate
from 18.04 to 20.04, so I am migrating preventively.

.github/workflows/validate-header-guards.yml

@@ -5,7 +5,7 @@ on: [push, pull_request]
 jobs:
   build:
     # Set the type of machine to run on
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
 
       - name: Checkout source code