Compiler override no longer works

[snoyberg]

Setting compiler: on master does not cause a different compiler to be used. This does work on Stack 1.9.3. I've already created an integration test to demonstrate the problem:

• https://github.com/commercialhaskell/stack/compare/compiler-override-does-not-work?expand=1
• 59086c1

CC @qrilka

[snoyberg]

Fixed by #4608