Dawn build: find the correct version of ninja.

Use shutil.which() to find ninja, as is done
for cmake in the other Dawn build scripts.

Bug: 457463690

Change-Id: I5bf6be3afb3bb79ae618858a683258cc57310132
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1088016
Reviewed-by: Kaylee Lubick <kjlubick@google.com>
Commit-Queue: Stephen White <senorblanco@chromium.org>
Reviewed-by: Greg Daniel <egdaniel@google.com>