corrected spirv tools output file name

The incorrect output file name meant that the update_build_version.py script
was being run on every build, resulting in unnecessary work.

Bug: skia:
Change-Id: I5da933fb9b8fdf0bc898d0e90eaae0a542cfb3b3
Reviewed-on: https://skia-review.googlesource.com/50302
Reviewed-by: Cary Clark <caryclark@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
1 file changed