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.
Github-Pull: #6269
Rebased-From: c110575a92ebe2e9a58b53d56aafa1f1ae37dbb2
Cory Fields