chore: do not run the backporting workflow on forks (#22836)

I'm using my own fork of the scala3 repo (`hamzaremmal`) and each time I
sync it from the upstream (`scala/scala3`) I start having CI errors.

This PR changes the behaviour of the backporting workflow to only run on
`scala/scala3` as it should.

[skip ci]

Author: hamzaremmal

.github/workflows/lts-backport.yaml

@@ -7,7 +7,8 @@ on:
 
 jobs:
   add-to-backporting-project:
-    if: "!contains(github.event.push.head_commit.message, '[Next only]')"
+    if: "!contains(github.event.push.head_commit.message, '[Next only]') &&
+         github.repository == 'scala/scala3'"
     runs-on: ubuntu-latest
 
     steps: