gitian: Use the new bitcoin-detached-sigs git repo for OSX signatures

Rather than fetching a signature.tar.gz from somewhere on the net, instruct
Gitian to use a signature from a tag in the bitcoin-detached-sigs repository
which corresponds to the tag of the release being built.

This changes detached-sig-apply.sh to take a dirname rather than a tarball as
an argument, though detached-sig-create.sh still outputs a tarball for
convenience.