Use lower case for build folder name
Use lower case for build folder name

Previous qibuild version was creating a
build-<config>-release when building in release, so
we should keep this behavior.

Fixes #7892

(And it looks nicer)

Change-Id: Icba9dc3b62dc7fe0626eda747832774934685365

aldebaran/qibuild,dmerejkowsky/qibuild,aldebaran/qibuild,aldebaran/qibuild,dmerejkowsky/qibuild,aldebaran/qibuild,dmerejkowsky/qibuild,dmerejkowsky/qibuild,dmerejkowsky/qibuild