ci/deqp-runner: fix --fraction-start in non-parallel jobs

`CI_NODE_INDEX` is only defined in `parallel:` jobs.

Without this, we end up with `--fraction-start --fraction 1`, which is
obviously invalid but somehow it hasn't blown up (yet).

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/30462>

.gitlab-ci/cuttlefish-runner.sh

@@ -99,7 +99,7 @@ $ADB shell "mkdir /data/results; cd /data; ./deqp-runner \
     --skips /data/all-skips.txt $DEQP_SKIPS \
     --flakes /data/$GPU_VERSION-flakes.txt \
     --testlog-to-xml /deqp/executor/testlog-to-xml \
-    --fraction-start $CI_NODE_INDEX \
+    --fraction-start ${CI_NODE_INDEX:-1} \
     --fraction $(( CI_NODE_TOTAL * ${DEQP_FRACTION:-1})) \
     --jobs ${FDO_CI_CONCURRENT:-4} \
     $DEQP_RUNNER_OPTIONS"

.gitlab-ci/deqp-runner.sh

@@ -203,7 +203,7 @@ else
         --skips $INSTALL/all-skips.txt $DEQP_SKIPS \
         --flakes $INSTALL/$GPU_VERSION-flakes.txt \
         --testlog-to-xml /deqp/executor/testlog-to-xml \
-        --fraction-start $CI_NODE_INDEX \
+        --fraction-start ${CI_NODE_INDEX:-1} \
         --fraction $((CI_NODE_TOTAL * ${DEQP_FRACTION:-1})) \
         --jobs ${FDO_CI_CONCURRENT:-4} \
         $DEQP_RUNNER_OPTIONS