ci: only run vale for on pull requests

Signed-off-by: David Karlsson <35727626+dvdksn@users.noreply.github.com>

.github/workflows/build.yml

@@ -36,6 +36,7 @@ jobs:
             *.cache-to=type=gha,scope=build,mode=max
 
   vale:
+    if: ${{ github.event_name == 'pull_request' }}
     runs-on: ubuntu-22.04
     steps:
       - uses: actions/checkout@v4