Add flyctl setup step and use bundle exec for rake (no binstubs in CI).

https://claude.ai/code/session_01Bgey3hrVy9av1SHrpdo5bU

diff --git a/.github/workflows/preview.yml b/.github/workflows/preview.yml
index 35f9b38..1b55f80 100644
--- a/.github/workflows/preview.yml
+++ b/.github/workflows/preview.yml
@@ -28,13 +28,15 @@ jobs:
       - name: Capture snapshot diff
         if: github.event.action != 'closed'
         run: |
-          rake snapshots:diff
+          bundle exec rake snapshots:diff
           snapshot_dir="${XDG_CACHE_HOME:-$HOME/.cache}/ketchup/snapshots"
           cp -r "$snapshot_dir" public/snapshots
           cp public/favicon.svg public/snapshots/
         env:
           GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
 
+      - uses: superfly/flyctl-actions/setup-flyctl@master
+
       - name: Deploy preview
         id: deploy
         uses: superfly/fly-pr-review-apps@1.5.0