properly detect msys_nt as windows in the ninja script

Some versions of bash for windows report their uname -s as "MSYS_NT-10.0-{VERSION}",
this updates the wrapper script to handle them.

Change-Id: I5916324fc93d86d71f97ae3ad8497a5bb11a350c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1941341
Auto-Submit: Samuel Attard <samuel.r.attard@gmail.com>
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>

ninja

@@ -37,6 +37,7 @@ case "$OS" in
   Darwin)    exec "${THIS_DIR}/ninja-mac" "$@";;
   CYGWIN*)   exec cmd.exe /c $(cygpath -t windows $0).exe "$@";;
   MINGW*)    cmd.exe //c $0.exe "$@";;
+  MSYS_NT*)  cmd.exe //c $0.exe "$@";;
   *)         echo "Unsupported OS ${OS}"
              print_help
              exit 1;;