Fix dmg2mar after dmg changes from #28124

While signing 12.0.3, the dmg2mar step failed with:

2023-02-17T13:51:16+00:00 - Starting step: dmg2mar
renamed '/home/richard/tor-browser-build/tools/signing/../../release/signed/12.0.3-macos-signed/TorBrowser-12.0.3-macos_ALL.dmg' -> '/home/richard/tor-browser-build/tools/signing/../../release/signed/12.0.3/TorBrowser-12.0.3-macos_ALL.dmg'
git submodule update --init
./rbm/rbm build release --step update_responses_config --target release --target signed
./rbm/rbm build release --step dmg2mar --target release --target signed
Starting TorBrowser-12.0.3-macos_ALL.dmg -> tor-browser-macos-12.0.3_ALL.mar
Can't stat /home/richard/tor-browser-build/tmp/rbm-kVz_03/zPDj49D69O/Tor Browser.app: No such file or directory
 at /home/richard/tor-browser-build/tools/dmg2mar line 142.
Error: Error updating tor-browser-macos-12.0.3_ALL.mar: /home/richard/tor-browser-build/tmp/rbm-kVz_03/wwW3_TJYtJ/mar-tools/make_full_update.sh: line 61: pushd: /home/richard/tor-browser-build/tmp/rbm-kVz_03/zPDj49D69O/Tor Browser.app: No such file or directory

From IRC:

< PieroV> We've switched to HFS, and 7z handles the extraction in a slightly different way
< PieroV> It adds an additional "Tor Browser" directory (which happens to be the volume name)