This standardizes the use of
$APK as well as adding some other interposition variables:
$pkgbasedirfor easy out-of-aports.git builds
This allows APK Foundry to build packages outside of a git tree easily (by setting
git -C ...) as well as make it easier to enforce network isolation while poking holes for source file and dependency downloading.
An older form of this patch set has been in use for some time on my personal machines while developing the project, and it was recently merged into Adélie's abuild.git as we prepare to setup CI using it.