actually make variables are exported if we use the keyword "export". Might be worth it to try it using target-specific var, for more clarity (althought that's something we can discuss)

That would be:
```makefile
.PHONY: test-perf
test-perf: export APPS MARIONETTE_RUNNER_HOST NPM_REGISTRY
test-perf:
  GAIA_DIR="`pwd`" ./bin/gaia-perf-marionette
```

Not sure at all this works, but if this works, this is more readable to me. What do you think?