For posterity, this is a bash/terminal command for mac OS (all on one line) I used to download ISRG Root X1 and install it:
curl http://x1.i.lencr.org/ --output $HOME/Downloads/ISRG_Root_X1.der | sudo security add-trusted-cert -d -r trustRoot -k "/Library/Keychains/System.keychain" "$HOME/Downloads/ISRG_Root_X1.der"