AGENT-974: AGENT-959 changes broke 'make check' in sdc-vm-agent.git
AGENT-974: AGENT-959 changes broke 'make check' in sdc-vm-agent.git

This fixes 'make check' on mac at least: don't bother building a local
node to NODE_PREBUILT_VERSION spec on Mac, just use what 'node' and
'npm' are on the path.

joyent/sdc-vm-agent,joyent/sdc-vm-agent