Run scheduled actions only on the main repo (#27343)

.github/workflows/build-nightly.yml

@@ -11,6 +11,7 @@ permissions:
 jobs:
   compute-suffix:
     runs-on: ubuntu-latest
+    if: github.repository == 'mastodon/mastodon'
     steps:
       - id: version_vars
         env:

.github/workflows/crowdin-download.yml

@@ -11,6 +11,7 @@ permissions:
 jobs:
   download-translations:
     runs-on: ubuntu-latest
+    if: github.repository == 'mastodon/mastodon'
 
     steps:
       - name: Checkout