ci: gate element-hq-only workflows on fork

Skip upstream deploy/publish/release, org automation, translations and
PR previews via a repo guard; allow PRs to release/* base branches.

.github/workflows/backport.yml

@@ -17,15 +17,7 @@ jobs:
         runs-on: ubuntu-24.04
         # Only react to merged PRs for security reasons.
         # See https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#pull_request_target.
-        if: >
-            github.event.pull_request.merged
-            && (
-              github.event.action == 'closed'
-              || (
-                github.event.action == 'labeled'
-                && contains(github.event.label.name, 'backport')
-              )
-            )
+        if: ${{ github.repository == 'element-hq/element-web' && (github.event.pull_request.merged && ( github.event.action == 'closed' || ( github.event.action == 'labeled' && contains(github.event.label.name, 'backport') ) )) }}
         steps:
             - uses: tibdex/backport@9565281eda0731b1d20c4025c43339fb0a23812e # v2
               with: