Allow building installables in 'attic push'

[Benjamin-L]

Currently, attic push can only be used with nix store paths. Many use-cases involve building a derivation and then pushing the built derivation closure. We're currently using a script mostly for the purpose of wrapping nix build .#foo && attic push $(nix path-info --derivation .#foo) .#foo in CI.

This script could be replaced if attic had support for

• building installables passed to attic push
• pushing the closure of a path's derivation in addition to the path itself (build-time deps in addition to runtime)