workflows/triage: fix excessive-permissions zizmor error

This addresses the `excessive-permissions` error from `zizmor` by
moving the workflow permissions to the jobs where they're needed.

.github/workflows/triage.yml

@@ -6,17 +6,16 @@ concurrency:
   group: "triage-${{ github.event.number }}"
   cancel-in-progress: true
 
-permissions:
-  contents: read
-  pull-requests: write
-
 jobs:
   limit-pull-requests:
     if: >
       always() && github.repository_owner == 'Homebrew' &&
       (github.event_name == 'pull_request_target' &&
        github.event.action == 'opened')
     runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      pull-requests: write
     steps:
       - uses: Homebrew/actions/limit-pull-requests@master
         with:
@@ -31,6 +30,9 @@ jobs:
 
   triage:
     runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      pull-requests: write
     steps:
       - name: Check commit format
         uses: Homebrew/actions/check-commit-format@master