vars.SKIP_END_TO_END optionally skip tests when building a release

The end-to-end tests will always fail when more than one release is
broken. When trying to fix one, the other will get in the way and vice
versa. The only way to get out of this deadlock is to replace all
broken releases but one by doing the following on forgejo-integration:

* set SKIP_END_TO_END to true in the actions vars tab
* pushing a commit to the corresponding branch, fixing the problem

(cherry picked from commit 54c8ac3e39)

.forgejo/workflows/build-release.yml

@@ -194,7 +194,7 @@ jobs:
           verbose: ${{ vars.VERBOSE || secrets.VERBOSE || 'false' }}
 
       - name: end-to-end tests
-        if: ${{ secrets.TOKEN != '' && vars.ROLE == 'forgejo-integration' }}
+        if: ${{ secrets.TOKEN != '' && vars.ROLE == 'forgejo-integration' && vars.SKIP_END_TO_END != 'true' }}
         uses: https://code.forgejo.org/actions/cascading-pr@v2
         with:
           origin-url: ${{ env.GITHUB_SERVER_URL }}