Consider reconfiguring if $PATH changes

[ndmitchell]

As per haskell/cabal#2015, it seems Cabal caches the $PATH when running configure and reuses it later. As a result, if a package needs a particular program on $PATH at compile time, a user who modifies their $PATH after a failure will continue to remain in a failure state until running a stack clean. Perhaps the $PATH should be considered an input dependency of configure steps?

[mgsloan]

Good to know! Yes, I think it makes sense to do a reconfigure. The alternative would be to just make this a warning, but that could easily be ignored. Marking as P1 since it has to do with build determinism

[mgsloan]

Fixing this will probably invalidate previous configure cache info, so ideally would be batched with any other pending changes to it. Would be possible to have support for reading the old cache version.

[snoyberg]

See #4740