I’ve already got the extra private key, so: yes.
Moreover, in order to apply the .insteadOf option, I need a configuration file. And, because I’m keeping my personal and work configurations separate, it’s a separate configuration file.
And, if I could persuade Mix.install to use a separate configuration file, I wouldn’t need the env var – because the configuration file already specifies core.sshCommand, but that doesn’t seem to do anything here either.
