build: fix test-gc weakref build rule

Make the build rule depend on the build artifact (weakref.node) itself
rather than the directory it's built in.  Depending on the directory
means that a build failure won't trigger a rebuild on the next
invocation because the directory's timestamp has been updated.

This is a back-port of commit 1189571 from the master branch that
hopefully fixes the following CI error:

    executing:  make test/gc/node_modules/weak/build/
    make: *** No rule to make target `test/gc/node_modules/weak/build/'.
    Command exited with non-zero: make test/gc/node_modules/weak/build/
    Build step 'Execute NodeJS script' marked build as failure