ci: Only run mutants workflow under certain conditions (#2158)

And simplify the per-step conditions accordingly.

.github/workflows/mutants.yml

@@ -17,6 +17,7 @@ permissions:
 
 jobs:
   mutants:
+    if: github.event_name == 'pull_request' || github.event_name == 'schedule' || github.event_name == 'workflow_dispatch'
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@d632683dd7b4114ad314bca15554477dd762a938 # v4.2.0
@@ -51,18 +52,16 @@ jobs:
           echo 'TITLE=All Mutants' >> "$GITHUB_ENV"
 
       - name: Post step summary
-        if: always()
         run: |
           {
             echo "### $TITLE"
             echo "See https://mutants.rs/using-results.html for more information."
             echo '```'
-            sed 's/\x1b\[[0-9;]*[mGKHF]//g' results.txt
+            sed 's/\x1b\[[0-9;]*[mGKHF]//g' results.txt || true
             echo '```'
           } > "$GITHUB_STEP_SUMMARY"
 
       - uses: actions/upload-artifact@50769540e7f4bd5e21e526ee35c689e35e0d6874 # v4.4.0
-        if: always()
         with:
           name: mutants.out
           path: mutants.out