ci: skip Bundler install in gem-push workflow

The Build + Publish run failed at ruby/setup-ruby
(https://github.com/browserstack/browserstack-local-ruby/actions/runs/36133351430).
Gemfile.lock now says BUNDLED WITH 2.7.1 (regenerated in ee74327), so by
default setup-ruby runs `gem install bundler -v 2.7.1`. Bundler 2.7 needs
Ruby >= 3.2, so the install fails on the job's Ruby 2.6.10:

  There are no versions of bundler (= 2.7.1) compatible with your Ruby & RubyGems
  bundler requires Ruby version >= 3.2.0.

This job only runs `gem build` and `gem push`, and neither uses Bundler.
Set `bundler: none` so setup-ruby skips the install. The publish Ruby stays
at 2.6.10. The lockfile keeps Bundler 2.7.1 and its CHECKSUMS block, because
downgrading to Bundler 2.4.x (the last line that supports Ruby 2.6) would drop
the 524db87 integrity hardening.

Verified: the workflow YAML parses; the pinned setup-ruby SHA (v1.324.0)
documents 'none' as "nothing is done"; `gem build *.gemspec` succeeds without
Bundler. The workflow itself was not run, since it publishes to RubyGems.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Author: 07souravkunda

.github/workflows/gem-push.yml

@@ -16,6 +16,10 @@ jobs:
       uses: ruby/setup-ruby@a0102e0972be65f351c307e2d64b9314a57c8073 # v1.324.0
       with:
         ruby-version: 2.6.10
+        # This job only runs `gem build` / `gem push`, so it doesn't need Bundler.
+        # Without this, setup-ruby installs the Gemfile.lock BUNDLED WITH version
+        # (2.7.1, which needs Ruby >= 3.2), and that fails on Ruby 2.6.
+        bundler: none
 
     - uses: rubygems/configure-rubygems-credentials@762a4b77c3300434bb57c7ce80b20e36231927aa # v2.0.0
     - name: Build and push gem