Generic builds: provide config for artifact subdir

[mistydemeo]

Right now, we assume that generic build artifacts are located at the root of the build directory. We should add a configuration value that configures an alternate root to locate artifacts.

I'm experimenting with packaging an autotools-based package, and for packages using this and cmake, it would be much simpler to be able to root cargo-dist's binaries inside a build or build/bin directory.