chore(linux): Don't set VERSION_TAG to …-local in .deb packages

Previously we didn't detect that we were building a .deb package on
Launchpad or a Debian server, so we set VERSION_TAG to end in `-local`.
This change now checks the `KEYMAN_PKG_BUILD` environment variable which
will be set during .deb package builds (set in `debian/rules`). The
downside of this solution is that now `-local` also gets omitted if
we build a .deb package on a local dev machine, but that probably doesn't
happen too often, and even less often with modified source code. The
majority of builds where we want `-local` are local builds that get
installed locally, and those cases are covered by this change. And what
is more important is that for release builds we no longer append `-local`.