Add required env variable to sonarcloud CI

This variable is now required, and builds will fail if it's not defined.

.github/workflows/sonarcloud.yml

@@ -8,6 +8,7 @@ jobs:
   sonarcloud:
     env:
       DATABASE_URL: 'postgis://postgres:postgres@localhost/github_actions'
+      PRIVATE_FILES_LOCATION: '/tmp/private_files'
     runs-on: ubuntu-latest
     strategy:
       matrix: