Bug 1070442 - Support building gaia apps in parallel

I'm not sure it is that reliable to set an env variable like that and running multiple processes. It may easily race.
If that works, it feels like being lucky. It would be better to pass BUILD_APP_DIR (and any other env variable that isn't the same for all apps, like appOptions.APP_DIR and STAGE_APP_DIR, via an argument to build-app.
```
function buildOneApp(module, appOptions) {
  ...
  '-e', 'run("' + module + '", "' + JSON.stringify(appOptions).replace(/\\/g, "\\\\").replace(/"/g, '\\"') + '");'
  ...
}
if (P)
  buildOneApp("build-app", appOptions);
else 
  require("build-app").execute(options, appOptions);
```
(This is just an example, feel free to do differently)