Fixes #10800, Fixes #9148: Ensure rubygems is loading gemrc properly on Windows

Prior to this commit, when Vagrant attempted to use the Gem library, it
would attempt to pass in a gemrc through an environment variable that
the rubygems library would try to split and parse. This is normally
fine, as the method in question would return empty if that file did not
exist. However if the user had a file that matches the drive that
Vagrant was installed on, rubygems would fail saying the folder was not
a file (or a gemrc, in this case).

This commit works around that by instead configuring the gemrc location
through ruby with `Gem.configuration`.

Related rubygems issue
[#2733](https://github.com/rubygems/rubygems/issues/2733)

lib/vagrant/bundler.rb

@@ -12,6 +12,8 @@ require "rubygems/name_tuple"
 require_relative "shared_helpers"
 require_relative "version"
 require_relative "util/safe_env"
+require_relative "util/platform"
+require "vagrant/util/subprocess"
 
 module Vagrant
   # This class manages Vagrant's interaction with Bundler. Vagrant uses
@@ -40,6 +42,17 @@ module Vagrant
     def initialize
       @plugin_gem_path = Vagrant.user_data_path.join("gems", RUBY_VERSION).freeze
       @logger = Log4r::Logger.new("vagrant::bundler")
+
+      # Because of a rubygems bug, we need to set the gemrc file path
+      # through this method rather than relying on the environment varible
+      # GEMRC. On windows, that path gets split on `:`: and `;`, which means
+      # the drive letter gets treated as its own path. If that path exists locally,
+      # (like having a random folder called `c` where the library was invoked),
+      # it fails thinking the folder `c` is a gemrc file.
+      gemrc_val = ENV["GEMRC"]
+      ENV["GEMRC"] = ""
+      Gem.configuration = Gem::ConfigFile.new(["--config-file", gemrc_val])
+      ENV["GEMRC"] = gemrc_val
     end
 
     # Enable Vagrant environment specific plugins at given data path