ci/zink: Only test half of piglit pre-merge on anv.

Unfortunately we're at 16 minutes of runtime for this job, which is much
too far past our guidelines.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25790>

src/gallium/drivers/zink/ci/deqp-zink-anv-tgl.toml

@@ -100,3 +100,4 @@ profile = "quick_gl"
 process_isolation = true
 skips = ["/install/zink-anv-tgl-premerge-skips.txt"]
 timeout = 180.0
+fraction = 2