EbuildMerge: add exit_hook to unlock build dir

EbuildMerge: add exit_hook to unlock build dir

This is needed since commit 7a6c34cc6bd6eb20a97e05347f87a0157f4ae58a in
order to ensure that the build dir is unlocked before exit listeners
are called. It fixes failures in catalyst stage 1 builds where two
instances of the same package, built sequentially for separate roots,
interfere with eachother's build dir lock.