murdock: prioritize job collection over build jobs

Previously, the fanned out job collection would add it's jobs to the
"default" queue, competing with build jobs. This could lead to a
situation where job collection would take very long or even time out.

This PR choses "default-first" as queue for the fan-out. Workers are
expected to add that queue *before* "default", so its jobs get processed
first.

.murdock

@@ -376,7 +376,7 @@ get_compile_jobs() {
 
     get_apps | \
         maybe_filter_changed_apps | \
-        dwqc ${DWQ_ENV} -s \
+        dwqc ${DWQ_ENV} --queue default-first -s \
         ${DWQ_JOBID:+--subjob} \
         "$0 get_app_board_toolchain_pairs \${1} $0 compile"
 }