Makefile: check for existence of dist in build-ui-ifne (#6119)

The build-ui-ifne target is responsible for deciding
whether the UI assets already exist on disk, or need
to be built by cloning the UI repo and running its build
scripts.

The old configuration would check whether the UI
repo folder existed, and assume that as long as this
folder exists, it can be used to embed the UI assets.
However, if the clone or ui build fails at any point, the
Makefile target isn't able to retry the script.

Correct the path that is checked for to be the path that
is actually included in the assets.go file. This way, the
Makefile script will retry building the UI until a dist
folder is created.

Makefile

@@ -128,7 +128,7 @@ clean-ui:
 
 .PHONY: build-ui-ifne
 build-ui-ifne:
-ifeq (,$(wildcard internal/ui/.tmp/boundary-ui))
+ifeq (,$(wildcard internal/ui/.tmp/boundary-ui/ui/admin/dist))
 	@echo "==> No UI assets found, building..."
 	@$(MAKE) build-ui
 else