Make minitest install optional

Author: soutaro

.github/workflows/ruby.yml

@@ -12,6 +12,7 @@ jobs:
     strategy:
       matrix:
         container_tag:
+        - master-nightly-focal
         - 3.0-focal
         - 2.7-bionic
         job:
@@ -31,6 +32,10 @@ jobs:
         ruby -v
         gem install bundler
         bin/setup
+    - name: Install minitest
+      run: |
+        bundle install --with minitest
+      if: "!contains(matrix.container_tag, 'master-nightly')"
     - name: Run test
       run: |
         bundle exec rake ${{ matrix.job }}

Gemfile

@@ -23,3 +23,7 @@ gem "rbs-amber", path: "test/assets/test-gem"
 group :ide, optional: true do
   gem "ruby-debug-ide"
 end
+
+group :minitest, optional: true do
+  gem "minitest"
+end