ci: Pin Bundler to 4.0.6 in RuboCop workflow as workaround for ruby/rubygems#9536

The RuboCop workflow started failing on master at 2026-05-12T01:01 UTC with:

    Could not find compatible versions

against the multi-gemspec git source `rails/rails` whose top-level gem is in
a filtered-out group when `BUNDLE_ONLY=rubocop bundle install` runs under
Ruby 4.0.4 (Bundler 4.0.11). The regression is tracked in
ruby/rubygems#9536; Bundler 4.0.5 and 4.0.6 succeed while 4.0.7+ fail.

Pin Bundler explicitly so the workflow keeps rolling on `ruby-version: "4.0"`:

- set BUNDLER_VERSION=4.0.6 at the job level so every bundle invocation
  resolves to that version
- gem install bundler -v 4.0.6 so the version is actually present (the
  ruby-builder tarball ships only one Bundler, and current 4.0.x tarballs
  no longer ship 4.0.6)

Drop both once Bundler ships a fix for ruby/rubygems#9536.

Author: yahonda

.github/workflows/rubocop.yml

@@ -10,13 +10,21 @@ jobs:
   build:
 
     runs-on: ubuntu-latest
+    env:
+      BUNDLER_VERSION: 4.0.6
 
     steps:
     - uses: actions/checkout@v6
     - name: Set up Ruby 4.0
       uses: ruby/setup-ruby@v1
       with:
         ruby-version: "4.0"
+    - name: Pin Bundler to 4.0.6 (workaround for ruby/rubygems#9536)
+      run: |
+        gem install bundler -v 4.0.6
+        ruby --version
+        gem --version
+        bundle --version
     - name: Build and run RuboCop
       run: |
         BUNDLE_ONLY=rubocop bundle install --jobs 4 --retry 3