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>