Fix release: bundler frozen mode and silent rake docs:build failure

[joelhawksley]

Follow-up to #2694. The last release workflow run 32759987499 reported success, but the resulting release PR (#2695) has stale docs because rake docs:build silently failed:

bundler: failed to load command: rake
The gemspecs for path gems changed, but the lockfile can't be updated
because frozen mode is set (Bundler::ProductionError)

Root cause

ruby/setup-ruby with bundler-cache: true enables Bundler's frozen mode. After script/release bumps the gemspec version, bundle (install) refuses to rewrite Gemfile.lock, leaving the lockfile pointing at the old version. bundle exec rake docs:build then aborts on the gemspec/lockfile mismatch. The script had no exit-code check on that step, so the release continued and pushed a PR with un-rebuilt docs.

Fix

• export BUNDLE_FROZEN=false and export BUNDLE_DEPLOYMENT=false at the top of the script so every bundler invocation can rewrite the lockfile after the version bump.
• Switch update_gemfiles from bundle (install) to bundle lock, matching what the appraisal loop already does — gems are already installed by setup-ruby; we only need to refresh the lockfile.
• Fail hard if script/sync_contributors.rb or rake docs:build returns non-zero, so a broken release can never be pushed again.

I skipped set -euo pipefail because the existing script relies on pipelines that intentionally return non-zero (e.g. grep "PRE = " ... | grep -o "\"[^\"]*\"" when PRE = nil); explicit checks on the failing step are safer than blanket strict mode here.