builder: env overwrite test no longer needs an escape

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
builder: TestEnvUsage was not populating HOME because it does not exist by default in the busybox image.

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)