Invoke rbs collection install before steep check

Author: mame

.github/workflows/test.yaml

@@ -35,6 +35,7 @@ jobs:
           ruby-version: ${{ matrix.ruby }}
           bundler-cache: true
       - run: bundle install
+      - run: bundle exec rbs collection install
       - run: bundle exec steep check
   test-ruby:
     runs-on: ubuntu-20.04

.gitignore

@@ -2,3 +2,4 @@
 .irbrc
 /Gemfile.lock
 /pkg/
+/.gem_rbs_collection/