YARN-11078. Set env vars in a cross platform compatible way (#4432)

* Maven runs the ember build script.
  The environment variable TMPDIR was
  set as per bash syntax.
* This failed on Windows since the
  Windows command prompt doesn't
  support bash syntax.
* We're now detecting the OS and
  setting a Maven property
  "emberBuildScript" in a cross
  platform compatible way.
2 files changed