build: simplify execution of built binary

Since we aleady have a variable with path to the newly built
binary, use that instead of prefixing path. This also allows us
to pass a different path through the environment (NODE=)

PR-URL: https://github.com/nodejs/io.js/pull/1955


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Rod Vagg <rod@vagg.org>