ci: fix Rails 8.0 upstream tests

which was failing because the bundler binstub has been removed in
modern bundler, see rubygems/rubygems#8345

Author: flavorjones

test/integration/user_install_test.sh

@@ -6,6 +6,7 @@ set -o pipefail
 set -eux
 
 # set up dependencies
+gem install bcrypt # it's complicated, see Rails 7549ba77. can probably be removed once Rails 8.0 is EOL.
 rm -f Gemfile.lock
 bundle remove actionmailer || true
 bundle remove rails || true