Check `if [[ $(uname -s) = MINGW* ]]` instead of using
grepping and an additional variable.

Change-Id: I6f2cfbb6743357125e6672b77667ee5e36d3d81e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2127674
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Josip Sokcevic <sokcevic@google.com>