Just so you guys know, I’ve been hit by this today:
https://bugs.launchpad.net/ubuntu/+source/torbrowser-launcher/+bug/1678355
@popey found a workaround and posted it in the comments on the bug report:
gpg --homedir “$HOME/.local/share/torbrowser/gnupg_homedir/” --refresh-keys --keyserver pgp.mit.edu