Add step to install dependencies in release workflow (#730)

Sorry, I made a mistake in the previous PR because I had assumed that `bundler-cache: false` would still install dependencies. 

Apparently, it does not. So we have to install them explicitly (while continuing to not do caching, to protect against cache poisoning).

Author: connorshea

.github/workflows/push_gem.yml

@@ -29,6 +29,8 @@ jobs:
         with:
           bundler-cache: false
           ruby-version: ruby
+      - name: Install dependencies
+        run: bundle install
       - name: Release
         run: bundle exec rake release
       - name: Wait for release to propagate