get-win32-tarballs: Use correct `find`

Authored by bgamari on Sep 4 2017, 7:01 AM.

Description

get-win32-tarballs: Use correct find

This fixes Trac #12502 by using the find utility found by FP_PROG_FIND
instead of the first one in PATH.

Test Plan: Validate on Windows

Reviewers: Phyx, austin, hvr

Reviewed By: Phyx

Subscribers: rwbarton, thomie, erikd

GHC Trac Issues: Trac #12502

Differential Revision: https://phabricator.haskell.org/D3907