Hi,

When 'guix shell' is run without arguments, there is some convenient
default logic applied to check for a manifest.scm or guix.scm file and do
the right thing with it. However, using -- to override the default command
like 'guix shell -- make' doesn't do the same thing. I expect that it would
still automagically apply manifest.scm or guix.scm but just run the
specified command instead of spawning a shell.  Instead, 'guix shell'
outputs this warning letting me know that something isn't right:

    guix shell: warning: no packages specified; creating an empty
environment

On one hand: Sure, I *did* pass arguments (though not flags.) On the other
hand: I think this is a bad user experience. I doubt I'm alone in expecting
the only difference between 'guix shell' and 'guix shell -- make' to be
that 'make' is run instead of a shell.  I can implement this if there's
some indication that such a patch would be acceptable.

Thoughts?

- Dave