[ruby/rubygems] Skip checksum for the bundler gem if no bundler.gem exists on disk:

- ### Problem

  This change is purely to fix a problem when developing Bundler.
  When we tried to release Bundler 4.0.9, we bumped the VERSION
  from `4.1.0.dev` to `4.0.9`, this condition now evaluates to false
  https://github.com/ruby/rubygems/blob/34d19fa8a3f84c50e2ba65e0f39c80045e7cbfb8/bundler/lib/bundler/lockfile_generator.rb#L106.
  We then ended up with a CI crash.

  ```
  Errno::ENOENT: No such file or directory @ rb_sysopen - /Users/runner/work/rubygems/rubygems/cache/bundler-4.0.9.gem
  ```

  ### Context

  Computing a checksum for the `bundler` gem is only possible
  when the `bundler.gem` binary exists on disk. When a regular
  user interacts with `bundler`, the spec is loaded from disk and
  an associated cached `bundler.gem` should exists.

  However, when developing Bundler, the spec doesn't come from disk
  but from a fake spec https://github.com/ruby/rubygems/blob/34d19fa8a3f84c50e2ba65e0f39c80045e7cbfb8/bundler/lib/bundler/source/metadata.rb#L22-L28
  that with no associated `bundler.gem`.

  ### Solution

  To prevent CI from breaking whenever we make a release, we have
  to skip computing a checksum if no `bundler.gem` exists.

ruby/rubygems@01e0e61612

Author: Edouard-chin

lib/bundler/lockfile_generator.rb

@@ -105,9 +105,11 @@ def add_section(name, value)
     def bundler_checksum
       return [] if Bundler.gem_version.to_s.end_with?(".dev")
 
+      bundler_spec = definition.sources.metadata_source.specs.search(["bundler", Bundler.gem_version]).last
+      return [] unless File.exist?(bundler_spec.cache_file)
+
       require "rubygems/package"
 
-      bundler_spec = definition.sources.metadata_source.specs.search(["bundler", Bundler.gem_version]).last
       package = Gem::Package.new(bundler_spec.cache_file)
       definition.sources.metadata_source.checksum_store.register(bundler_spec, Checksum.from_gem_package(package))