Add option to setup system subnet

[nathanosdev]

I commonly find myself needing to setup a system subnet for DFX (for use with dfx nns). It would be nice to have that encapsulated inside this action. The step I use is this, it could easily be configured further to only run when an input variable setupSystemSubnet is set to true.

    - name: Configure system subnet
      shell: bash
      run: |
        mkdir -p $HOME/.config/dfx
        cat <<EOF >$HOME/.config/dfx/networks.json
        {
          "local": {
            "bind": "127.0.0.1:8080",
            "type": "ephemeral",
            "replica": {
              "subnet_type": "system"
            }
          }
        }
        EOF

Full example of the action I use is here.