Add size and hash info during build

Previously, this was added during the report step, for extra
flexibility. However, adding it during the build already allows cleaning
up the build dir after building to not waste tons of disk space.
Optionally delete the build directory after building

This saves disk space for files which aren't normally needed anyway. If
needed, you can pass --keep-build-dir to keep them around (e.g. for
manual inspection).