ci: Stop triage-followup workflow from running on forks (#4629)

## Changes

This workflow only seems relevant on the main repository. Add a
condition to exclude it from forks.

This came up when I noticed this workflow failed on my fork yesterday
and today.

Co-authored-by: Armin Ruech <[email protected]>

Author: kaylareopelle

.github/workflows/triage-followup.yml

@@ -9,6 +9,7 @@ permissions:
 
 jobs:
   followup:
+    if: ${{ github.repository == 'open-telemetry/opentelemetry-specification' }}
     permissions:
       issues: write # required for adding triage labels to issues
     runs-on: ubuntu-latest