Use local gems instead of installing from web when deploying, oops!

I hadn't realized for a while that we weren't already doing this lol, I
had noticed that `bundle install` in production was slower than I
expected when adding new stuff, but it was when we did this big recent
`bundle update` that I really noticed the difference.

Fixed now, I think! Though the real test will come when we actually
have a new gem to install, since this was a no-op case.

deploy/deploy.yml

@@ -78,7 +78,10 @@
     - name: Run `bundle install` to install dependencies in remote folder
       command:
         chdir: "{{ remote_app_root }}"
-        cmd: /opt/ruby-3.3.0/bin/bundle install
+        # The `--local` flag instructs Bundler to use the cached dependencies
+        # in `vendor/cache`, instead of reading from the web, which is much
+        # faster and more reliable!
+        cmd: /opt/ruby-3.3.0/bin/bundle install --local
 
     - name: Update the `current` folder to point to the new version
       file: