[build] Fix RBE fallback on pkg, ddc and dyn bots.

Also fix --check-clean. `ninja target -t commands` reports the commands for the default target, fix to use `ninja -t commands target` instead.

Change-Id: Ice179290e75b083a96abadb7540ca960eafcc982
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/510440
Reviewed-by: Alexander Aprelev <aam@google.com>
Commit-Queue: Ryan Macnak <rmacnak@google.com>

utils/dynamic_module_runner/BUILD.gn

@@ -15,7 +15,8 @@ aot_snapshot("dynamic_module_runner_snapshot") {
   name = "dynamic_module_runner"
   gen_kernel_args =
       [ "--dynamic-interface=" + rebase_path(
-            "$_dart_root/utils/dynamic_module_runner/dynamic_interface.yaml") ]
+            "$_dart_root/utils/dynamic_module_runner/dynamic_interface.yaml",
+            root_build_dir) ]
 }
 
 aot_snapshot("dynamic_modules_test_suite_snapshot") {