release: create a bundle for the new signing script

Also change the mac filename to match

The procedure remains the same, but now there's a nifty script to automate
the signing process.

Future steps:
- Build osslsigncode in the gitian-win descriptor so that the signer itself is
  deterministic.
- Verify in the gitian-win-signer descriptor that the expected cert chain was
  used.

contrib/macdeploy/detached-sig-create.sh

@@ -10,7 +10,7 @@ BUNDLE="${ROOTDIR}/Bitcoin-Qt.app"
 CODESIGN=codesign
 TEMPDIR=sign.temp
 TEMPLIST=${TEMPDIR}/signatures.txt
-OUT=signature.tar.gz
+OUT=signature-osx.tar.gz
 OUTROOT=osx
 
 if [ ! -n "$1" ]; then