intel/dump_gpu: Fix name of LD_PRELOAD in env append logic

Checking for the wrong environment variable name to be set causes
us to stomp any pre-existing LD_PRELOAD.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4970>
1 file changed