ci: Terminate capture-devcoredump before tarring up artifacts.

Every few weeks we'd get a flake where the script noticed the devcore
right as we were wrapping up, and then tar would exit because the file it
was tarring changed underneath it.  Just kill devcores before we do that
-- even if we kill it while it's working, it's so rare that it probably
won't bother anyone.

Fixes: #5867
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16820>

.gitlab-ci/common/init-stage2.sh

@@ -4,11 +4,15 @@
 # exiting, since any console output may interfere with LAVA signals handling,
 # which based on the log console.
 cleanup() {
+  if [ "$BACKGROUND_PIDS" = "" ]; then
+    return 0
+  fi
+
   set +x
   echo "Killing all child processes"
   for pid in $BACKGROUND_PIDS
   do
-    kill "$pid"
+    kill "$pid" 2>/dev/null || true
   done
 
   # Sleep just a little to give enough time for subprocesses to be gracefully
@@ -18,6 +22,9 @@ cleanup() {
   do
     kill -9 "$pid" 2>/dev/null || true
   done
+
+  BACKGROUND_PIDS=
+  set -x
 }
 trap cleanup INT TERM EXIT
 
@@ -135,6 +142,11 @@ mv -f ${CI_PROJECT_DIR}/results ./ 2>/dev/null || true
 
 [ ${EXIT_CODE} -ne 0 ] || rm -rf results/trace/"$PIGLIT_REPLAY_DEVICE_NAME"
 
+# Make sure that capture-devcoredump is done before we start trying to tar up
+# artifacts -- if it's writing while tar is reading, tar will throw an error and
+# kill the job.
+cleanup
+
 # upload artifacts
 if [ -n "$MINIO_RESULTS_UPLOAD" ]; then
   tar -czf results.tar.gz results/;