Skip bundler self-checksum for unreleased bundlers

Using `Bundler.gem_version.end_with?(".dev")` only skips the own
checksum on master, but patch releases run from a source checkout
(e.g., bumping bundler/lib/bundler/version.rb to 4.0.11 on a release
branch) still record the checksum, which is environment dependent on
the local gem cache and causes frozen-lock drift on CI.

Generalize the guard with `released_bundler?`, which returns false for
any prerelease version and for bundlers loaded outside of an installed
gem location (`/specifications/`), so dev workflows don't record
self-checksums while released installs still do.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hsbt

bundler/lib/bundler/lockfile_generator.rb

@@ -103,7 +103,7 @@ def add_section(name, value)
     end
 
     def bundler_checksum
-      return [] if Bundler.gem_version.to_s.end_with?(".dev")
+      return [] unless released_bundler?
 
       bundler_spec = definition.sources.metadata_source.specs.search(["bundler", Bundler.gem_version]).last
       return [] unless File.exist?(bundler_spec.cache_file)
@@ -115,5 +115,11 @@ def bundler_checksum
 
       [definition.sources.metadata_source.checksum_store.to_lock(bundler_spec)]
     end
+
+    def released_bundler?
+      return false if Bundler.gem_version.prerelease?
+      # Released gem specs live under .../specifications/; source checkouts don't.
+      Gem.loaded_specs["bundler"]&.loaded_from.to_s.include?("/specifications/")
+    end
   end
 end