hadrian: handle findExecutable "" gracefully

hadrian may invoke findExecutable "" at run-time due to a certain
program is not found by configure script. Which is fine and
findExecutable is supposed to return Nothing in this case. However, on
Windows there's a directory bug that throws an exception (see
https://github.com/haskell/directory/issues/180), so we might as well
use a wrapper for findExecutable and handle exceptions gracefully.

(cherry picked from commit 1e5752f6)