Also fix scripts/gen_vk_gl_cts_build.py to always use '/' as file
separator. Otherwise, running the script on Windows uses '\' which
Linux doesn't like.

Test:
  angle_deqp_gles31_tests --use-angle=swiftshader --gtest_filter=dEQP.GLES31/functional_debug_negative_coverage_get_error_vertex_array_draw_range_elements

Bug: angleproject:2324
Bug: angleproject:4234
Change-Id: I3b212e0d0ba97996f1d86f7fc81e58c3b03a9cbb
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1975020
Commit-Queue: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>