build: don't rebuild test/gc add-on unnecessarily

Remove the explicit dependency on $(NODE_EXE), it always triggers a
rebuild due to it being a .PHONY rule.  Add-ons in test/addons/ have
the same implicit dependency so it isn't completely without precedent.

PR-URL: https://github.com/nodejs/node/pull/11311


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>