Replace #9506 with a targeted :ruby_repo skip

spec/runtime/setup_spec.rb

@@ -1309,7 +1309,12 @@ def lock_with(ruby_version = nil)
     end
 
     context "is not present" do
-      it "does not change the lock" do
+      # Skipped on ruby-core because `ruby "require 'bundler/setup'"` does not
+      # activate bundler as a gem there, so Source::Metadata falls back to a
+      # synthetic spec whose cache_file does not exist on disk and
+      # LockfileGenerator#bundler_checksum drops the bundler checksum, while
+      # the on-disk lockfile still has it.
+      it "does not change the lock", :ruby_repo do
         expect { ruby "require 'bundler/setup'" }.not_to change { lockfile }
       end
     end

spec/support/checksums.rb

@@ -58,12 +58,7 @@ def checksums_section(enabled = true, bundler_checksum: true, &block)
       ChecksumsBuilder.new(enabled, &block).tap do |builder|
         next if builder.bundler_registered || !bundler_checksum
 
-        # Mirror the conditions under which LockfileGenerator#bundler_checksum
-        # gives up and omits the bundler checksum from the lockfile:
-        #   - .dev: development builds have no released bundler.gem to checksum.
-        #   - ruby_core?: bundler is loaded as a default gem, so bundler.gem
-        #     is not present on disk under the test gem cache.
-        next if Bundler::VERSION.to_s.end_with?(".dev") || Spec::Path.ruby_core?
+        next if Bundler::VERSION.to_s.end_with?(".dev")
         builder.checksum(system_gem_path, "bundler", Bundler::VERSION, Gem::Platform::RUBY, "cache")
       end
     end