.github/workflows: Configure warnings-as-errors policy

Enable WERROR=y for CI and package builds so warnings in SCST code
are fatal.

Keep WERROR=n for regression and Coverity because these workflows
must tolerate failures outside strict SCST package gates.

.github/workflows/run_regression_tests.yaml

@@ -8,6 +8,9 @@ on:
     branches:
       - master
 
+env:
+  WERROR: 'n'
+
 jobs:
   prepare_matrix:
     name: Prepare kernel matrix