chore(ci): disable running release-plz on forked repositories (#1730)

See https://github.com/jdssl/ratatui/pull/1#issuecomment-2739366609

.github/workflows/release-plz.yml

@@ -15,6 +15,7 @@ jobs:
   release-plz-release:
     name: Release-plz release
     runs-on: ubuntu-latest
+    if: ${{ github.repository_owner == 'ratatui' }}
     steps:
       - name: Checkout repository
         uses: actions/checkout@v4
@@ -34,6 +35,7 @@ jobs:
   release-plz-pr:
     name: Release-plz PR
     runs-on: ubuntu-latest
+    if: ${{ github.repository_owner == 'ratatui' }}
     concurrency:
       group: release-plz-${{ github.ref }}
       cancel-in-progress: false