ci: move script: override from .piglit-traces-test to llvmpipe-traces

This is already being set as needed everywhere else, and would cause
issues in future work.

Use the relative `install/` path for `HWCI_TEST_SCRIPT` as that's
supported by both HW runners and FDo runners.
A separate MR will fix the `/install/` vs `install/` mess.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/36145>

.gitlab-ci/test/gitlab-ci-inc.yml

@@ -143,12 +143,7 @@
     PIGLIT_REPLAY_DEVICE_NAME: $GPU_VERSION
     PIGLIT_RESULTS: "${GPU_VERSION}-replay"
     PIGLIT_NO_WINDOW: 1
-    HWCI_TEST_SCRIPT: "/install/piglit/piglit-traces.sh"
-  script:
-    - section_start variables "Variables passed through:"
-    - filter_env_vars
-    - section_end variables
-    - install/piglit/piglit-traces.sh
+    HWCI_TEST_SCRIPT: "install/piglit/piglit-traces.sh"
 
 .deqp-test:
   script: