workflows/periodic-merge: move fork condition to calling workflow

Apparently the skipping needs to happen on the calling workflow,
otherwise those workflows show up as "failed" in forks instead of
skipped.

.github/workflows/periodic-merge-24h.yml

@@ -20,6 +20,7 @@ permissions:
 
 jobs:
   periodic-merge:
+    if: github.repository_owner == 'NixOS'
     strategy:
       # don't fail fast, so that all pairs are tried
       fail-fast: false

.github/workflows/periodic-merge-6h.yml

@@ -20,6 +20,7 @@ permissions:
 
 jobs:
   periodic-merge:
+    if: github.repository_owner == 'NixOS'
     strategy:
       # don't fail fast, so that all pairs are tried
       fail-fast: false

.github/workflows/periodic-merge.yml

@@ -14,7 +14,6 @@ on:
 
 jobs:
   merge:
-    if: github.repository_owner == 'NixOS'
     runs-on: ubuntu-24.04
     name: ${{ inputs.from }} → ${{ inputs.into }}
     steps: