Disable update-dependencies on forks

This disables the update-dependencies cron job in forks. It's not
uncommon for people to leave GitHub Actions enabled in a fork (which in
my experience seems to be the default?), and this unfortunately means
that this job will run in all those forks which is probably not what
people want.

.github/workflows/update-dependencies.yml

@@ -8,6 +8,7 @@ jobs:
   update:
     name: Update dependencies
     runs-on: ubuntu-latest
+    if: github.repository == 'rust-lang/mdBook'
     steps:
       - uses: actions/checkout@v5
       - name: Install Rust