issue 2495: warn users when install yarn globally with yarn

[samjacobclift]

Summary
Built upon this original PR from mnsn

Add a warning when a user is installing Yarn globally with Yarn, see the issue for more details

Test plan

Do we need tests for this warning?

I've build yarn locally and used that binary to install yarn globally and I get the warning. Should we add tests for this warning? wdwt

[bestander]

Thanks!
Screenshot is a good test plan for this case

[samjacobclift]

@bestander thank you for the merge :)