Use bundler-cache to bundle install and cache gems

The setup-ruby action makes it trivial to bundle install gems from a
cache and update the cache when the run completes.

When using `bundler-cache`, we can't use `bundle config` commands, but
we can achieve the same effect specifying BUNDLE_SET as an env variable.

See https://github.com/ruby/setup-ruby for details

Author: joshcooper

.github/workflows/rspec_tests.yaml

@@ -24,6 +24,8 @@ jobs:
           - {os: windows-2019, ruby: '3.2'} # openssl 3
 
     runs-on: ${{ matrix.cfg.os }}
+    env:
+      BUNDLE_SET: "without packaging documentation"
     steps:
       - name: Checkout current PR
         uses: actions/checkout@v4
@@ -32,12 +34,7 @@ jobs:
         uses: ruby/setup-ruby@v1
         with:
           ruby-version: ${{ matrix.cfg.ruby }}
-
-      - name: Update rubygems and install gems
-        run: |
-          gem update --system --silent --no-document
-          bundle config set without packaging documentation
-          bundle install --jobs 4 --retry 3
+          bundler-cache: true
 
       - name: Run tests on Windows
         if: runner.os == 'Windows'