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.

contrib/macdeploy/detached-sig-apply.sh

@@ -20,7 +20,7 @@ fi
 
 rm -rf ${TEMPDIR} && mkdir -p ${TEMPDIR}
 tar -C ${TEMPDIR} -xf ${UNSIGNED}
-tar -C ${TEMPDIR} -xf ${SIGNATURE}
+cp -rf "${SIGNATURE}"/* ${TEMPDIR}
 
 if [ -z "${PAGESTUFF}" ]; then
   PAGESTUFF=${TEMPDIR}/pagestuff