fix(ci): install rsync in CI container

The node:20-bookworm image doesn't include rsync which is needed
for the sync-schemas npm script.

.forgejo/workflows/deploy-frontend.yml

@@ -61,6 +61,9 @@ jobs:
         working-directory: frontend
         run: pnpm install --frozen-lockfile
 
+      - name: Install rsync
+        run: apt-get update && apt-get install -y rsync
+
       - name: Sync LinkML schemas to frontend
         working-directory: frontend
         run: pnpm run sync-schemas