make release-LATEST release-$(PACKAGEVERSION) to upload package to GitHub as an asset of a release

GITHUB_REPO must be defined and release must be created in advance from GitHub
Shuts down incremental filesystem modification watch when the load is too high for a long period of time

watch-filesystem-events process staying over 100% for more than 20s will
stop the whole watch-modifications.  Also, more than 10 processes being
spawned will trigger the shutdown.

TODO: need a way to stop/pause watch-filesystem-events while BuildKit's
own build/stage and to resume later.