ci: Unify {BM,LAVA}_START_XORG environment

Why were they ever different ... ?

Signed-off-by: Daniel Stone <daniels@collabora.com>
Acked-by: Martin Peres <martin.peres@mupuf.org>
Acked-by: Emma Anholt <emma@anholt.net>
Reviewed-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11337>

.gitlab-ci/lava/lava.yml.jinja2

@@ -116,7 +116,7 @@ actions:
           # LAVA_TEST_SCRIPT because we need to use xinit to start X (otherwise
           # without using -displayfd you can race with Xorg's startup), but xinit will eat
           # your client's return code
-          - "if [ -n $LAVA_START_XORG ]; then
+          - "if [ -n $HWCI_START_XORG ]; then
                echo 'touch /xorg-started; sleep 100000' > /xorg-script;
                env xinit /bin/sh /xorg-script -- /usr/bin/Xorg -noreset -s 0 -dpms -logfile /Xorg.0.log &
                for i in 1 2 3 4 5; do