Disable the use of windows agility sdk on Dawn builds.

Dawn now requires the windows agility sdk to build. Until we update our
systems to pull that sdk deps, just disable the use of it.

Change-Id: I70bea13567528202403cafd635288fc846e51461
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1381136
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>