[ruby/rubygems] Add a missing "require 'etc'" statement:

- Ref https://github.com/ruby/rubygems/pull/9171#discussion_r2658056892
- Referencing the constant Etc may raise a `NameError` depending on
  whether other Rubygems/Bundler codepath have evaluated a prior
  "require 'etc'". This can be reproduced with this Gemfile:

  ```ruby
  source "https://rubygems.org"

  gem 'prism', github: "ruby/prism"
  ```

https://github.com/ruby/rubygems/commit/5d435e6863

lib/rubygems/installer.rb

@@ -943,7 +943,12 @@ class Gem::Installer
   end
 
   def build_jobs
-    @build_jobs ||= Etc.nprocessors + 1
+    @build_jobs ||= begin
+                      require "etc"
+                      Etc.nprocessors + 1
+                    rescue LoadError
+                      1
+                    end
   end
 
   def rb_config