Disable push trigger for lint workflow

The event property github.event.push.before seems to not be supported
and therefore the workflow as is fails on main.

Ignoring the trigger means a slight loss in coverage but as it still
fully checks PRs we are mostly good.

Signed-off-by: Ralph Sennhauser <ralph.sennhauser@gmail.com>

.gitea/workflows/lint.yml

@@ -2,13 +2,12 @@
 # yamllint disable rule:line-length
 name: lint
 on:
-  - push
   - pull_request
 jobs:
   cppcheck:
     runs-on: ubuntu-latest
     env:
-      BASE_SHA: ${{ github.event.pull_request.base.sha || github.event.push.before }}
+      BASE_SHA: ${{ github.event.pull_request.base.sha }}
     steps:
       - uses: actions/checkout@v4