OSX diskimages need 0775 folder permissions

Avoids endless Gatekeeper warnings (#7085)

contrib/gitian-descriptors/gitian-osx-signer.yml

@@ -34,5 +34,5 @@ script: |
   tar -xf ${UNSIGNED}
   OSX_VOLNAME="$(cat osx_volname)"
   ./detached-sig-apply.sh ${UNSIGNED} signature/osx
-  ${WRAP_DIR}/genisoimage -no-cache-inodes -D -l -probe -V "${OSX_VOLNAME}" -no-pad -r -apple -o uncompressed.dmg signed-app
+  ${WRAP_DIR}/genisoimage -no-cache-inodes -D -l -probe -V "${OSX_VOLNAME}" -no-pad -r -dir-mode 0755 -apple -o uncompressed.dmg signed-app
   ${WRAP_DIR}/dmg dmg uncompressed.dmg ${OUTDIR}/${SIGNED}