dmg2mar step also takes care of copying the signed+stabled dmg to the signed directory

From IRC:

<richard> mystery solved (I think) re not signed macOS dmgs
<richard> so we have a do-all-signing signing script
<ma1> \o/ (both for mystery and misery)
<richard> which does all the signing
<richard> the script itself just does some env setup adn then invokes a bunch of other scripts
<richard> and works pretty well, it keeps track of which steps of succeeded and so you can (generally) resume if something breaks midway
<richard> well we haven't been generating mars for privacy browser builds because no updater
<richard> buuut
<richard> the step that copies the signed+stabled dmg to the dest directory is part of mar generation for macOS which doesn't get called
<richard> so not that scary

Just need to tweak the dmg2mar and gatekeeper-bundling steps I think.