Add issue write permissions to mutants cron job workflow (payjoin#676)

Our current default workflow permisssions do not allow for writing
issues but I had made a workflow that already had this in place so we
just had a failed workflow that did not properly create the issue as
expected. This change is in lieu of changing the global workflow
permissions.

Closes payjoin#672

Author: spacebear21

.github/workflows/cron-weekly-mutants.yml

@@ -6,6 +6,8 @@ on:
 jobs:
   cargo-mutants:
     runs-on: ubuntu-latest
+    permissions:
+      issues: write
     steps:
       - uses: actions/checkout@v4
       - uses: taiki-e/install-action@v2