Set $ACLOCAL_PATH in `stack exec` appropriately

As I found out in this Phabricator thread, stack exec [--no-ghc-package-path] bash doesn't set $ACLOCAL_PATH. I'm not sure why this is. When starting a regular mingw64 session or Git Bash (even stack exec mintty and choosing mingw64), the paths are set correctly.

Steps to reproduce

1. stack exec bash on a Windows machine (AFAICT any stack config will do)
2. Realise that ehco $ACLOCAL_PATH is empty

$ stack --version
Version 1.5.1, Git revision 600c1f01435a10d127938709556c1682ecfd694e x86_64 hpack-0.17.1