Changing backwards slashes to forward slashes apparently confuses
msys2/mingw magic path handling. I don't quite understand why, but this
fixes it.

Test Plan: on Windows, make sure PATH does not contain
'inplace/mingw/bin' (let the testsuite driver add it), then run: make
TEST='ghcilink003 ghcilink006'. Before this patch, it would fail.

Reviewed by: Phyx, bgamari, austin

Differential Revision: https://phabricator.haskell.org/D1343