Add [depot_user] to run tool builtins

Now that the depot_user procedure has found use in the depot_autopilot
it has become a convention.

Fix #3127

tool/run/depot.inc

@@ -238,6 +238,13 @@ proc _collect_from_depot { archives } {
 }
 
 
+##
+# Parse depot user from run tool arguments with a fallback to "genodelabs"
+#
+#
+proc depot_user {} { return [get_cmd_arg --depot-user genodelabs] }
+
+
 ##
 # Import binary and raw content of the specified depot archives into the run
 # directory of the scenario