fix, mysql `db:purge` respects `Rails.env`.

Previously this method always established a connection to the test database.
This resulted in buggy behavior when combined with other tasks like
`bin/rake db:schema:load`.

This was one of the reasons why #15394 (22e9a91189af2c4e6217a888e77f22a23d3247d1)
was reverted:

> I’ve replicated it on a new app by the following commands: 1) rails
  generate model post:title, 2) rake db:migrate, 3) rake
  db:schema:load, 4) rails runner ‘puts Post.first’. The last command
  goes boom. Problem is that rake db:schema:load wipes the database,
  and then doesn’t actually restore it. This is all on MySQL. There’s
  no problem with SQLite.

  -- DHH

https://github.com/rails/rails/commit/22e9a91189af2c4e6217a888e77f22a23d3247d1#commitcomment-6834245