The installation manual has used a dead key server recently, this works as of 12.21:

    gpg --keyserver keys.openpgp.org --recv-keys 409B6B1796C275462A1703113804BB82D39DC0E3 7D2BAF1CF37B13E2069D6956105BD0E739499BDB

It is meant to recognise an unprivileged user and install in ~/, but sometimes for me it fails trying to write to /usr, but you can force it like this:

    \curl -sSL https://get.rvm.io | bash -s stable --ruby --user-install --path=/home/slack/.rvm --auto-dotfiles

Restart your shell and

    source ~/.rvm/scripts/rvm
    
Then

    rvm list known
    # rvm install 3.0 # using "--ruby" above will install the latest stable
    rvm use 3.0 --default

When adding a new host, add the hostname to ''.zshrc'' for RVM setup (it tries to mod ''.profile'', which is not necessarily loaded by zsh).