Rubygems depends on /usr/bin/env

[cbaines]

Without /usr/bin/env existing, when rubygems installs gems, the shebang in the bin wrappers is invalid #! ruby.

As a workaround, when using GuixSD, adding /usr/bin/env can be done with this service.

(extra-special-file "/usr/bin/env"
                            (file-append coreutils "/bin/env"))

Ideally, using rubygems would be unnecessary, but as an intermediate step, it would be better if the shebangs were valid.