[rubygems/rubygems] Fix race condition on bundler's parallel installer

When installing in parallel, bundler creates several `Gem::Installer`
instances that run in parallel. These installers access the `@@all` class
variable of `Gem::Specification` concurrently.

If a concurrent thread calls `Gem::Specification.reset` (resetting
`@all` to `nil`) while another thread is running
`Gem::Specification._all` or another method that expects `@@all` to be
loaded and not `nil`, that can result in `Enumerable` methods being
called on `nil`, resulting in crashes.

I fix it by protecting the other concurrent access to the `@all`
variable.

https://github.com/rubygems/rubygems/commit/58b343c530

lib/rubygems/installer.rb

@@ -509,7 +509,7 @@ class Gem::Installer
   end
 
   def generate_plugins # :nodoc:
-    latest = Gem::Specification.latest_spec_for(spec.name)
+    latest = Gem::Installer.install_lock.synchronize { Gem::Specification.latest_spec_for(spec.name) }
     return if latest && latest.version > spec.version
 
     ensure_writable_dir @plugins_dir