Make guix commands pipeable

[pkill9]

I would like to be able to pipe files into guix commands.

Specifically the `guix system build` command, so I can build a system
configuration on a remote Guix system over SSH, i.e. `cat config.scm |
ssh <remote-system> guix system build -`, or perhaps using the
`--expression` flag which would make more sense, e.g. `cat config.scm |
ssh <remote-system> guix system build -e -`.

While you can currently just copy over a file and then use that, it
would be a little cleaner and more stateless to use a pipe.

What do other people think?