ci: Uprev piglit to 6a4be9e9946d ("piglit: NOTE! Default branch is now main")

Along with other new tests, brings in the perf improvement for
gl-1.3-texture-env so we can stop skipping it.

Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9806>
14 files changed