Add morgan's key to the setup account on the signing machine

The setup/administration of the signing machine is done with the following two steps:

• running tools/signing/machines-setup/upload-tbb-to-signing-machine, which creates a tarball from tor-browser-build current HEAD commit, upload it, with a few other tools like mar-tools and rcodesign, to the signing machine using the setup account and extract them in the /signing directory.

• running /singing/tor-browser-build/tools/signing/machines-setup/setup-signing-machine as root on the signing machine.

So admistration of the signing machine requires:

• having the ssh key in ~setup/.ssh/authorized_keys (which is done in tools/signing/machines-setup/setup-signing-machine)
• having the ssh key in ~root/.ssh/authorized_keys or having the root password

/cc @morgan