Fix a runtime error when running cgi in WEBrick http server.

[HirohitoHigashi]

I use Ruby 27 2.7.4-1 x64 msvcrt and running WEBrick.
When run the CGI program, I got a runtime error in msys2_installation.rb.

error message is here.

C:/Ruby27-x64/lib/ruby/site_ruby/2.7.0/ruby_installer/runtime/msys2_installation.rb:76:in `iterate_msys_paths': undefined method `split' for nil:NilClass (NoMethodError)

When running the CGI program, the ENV environment variables were rewritten to the values for CGI.

e.g.

{"GATEWAY_INTERFACE"=>"CGI/1.1", "HTTP_ACCEPT"=>"text/html,application..."}

[larskanis]

Makes sense. Thank you!