Merge #6840

6840: fix installing gems on _really_ old versions of RubyGems r=segiddins a=colby-swandale

### What was the end-user problem that led to this PR?

Installing gems on _really really_ old versions of RubyGems is not working. See #6839

### What was your diagnosis of the problem?

We were accessing an instance variable which did not exist

### What is your fix for the problem, implemented in this PR?

instantiate the instance variable ourselves.

### Why did you choose this fix out of the possible options?

This was the simplest solution that can be easily merged with `1-17-stable`

Fixes #6839


Co-authored-by: Colby Swandale <me@colby.fyi>

lib/bundler/rubygems_gem_installer.rb

@@ -10,6 +10,13 @@ def self.at(*args)
       end
     end
 
+    attr_reader :options
+
+    def initialize(gem, options = {})
+      @options = {}
+      super
+    end
+
     def check_executable_overwrite(filename)
       # Bundler needs to install gems regardless of binstub overwriting
     end