Tarball trees: Propagate lastModified

[edolstra]

Motivation

This makes tarball flakes behave consistently with GitHub/GitLab flakes, e.g. it makes

nix flake metadata https://github.com/NixOS/nix/archive/2.17.0.zip

return the same "Last modified" value as

nix flake metadata github:NixOS/nix/2.17.0

which is important for flakes that depend on lastModified to evaluate consistently.

Context

Checklist for maintainers

Maintainers: tick if completed or explain if not relevant

• agreed on idea
• agreed on implementation strategy
• tests, as appropriate
  • functional tests - tests/**.sh
  • unit tests - src/*/tests
  • integration tests - tests/nixos/*
• documentation in the manual
• documentation in the internal API docs
• code and comments are self-explanatory
• commit message explains why the change was made
• new feature or incompatible change: updated release notes

Priorities

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[cole-h]

Oh nice, I didn't realize the tarball + github flakes differed in this way. This was already in the --json output, so I suppose it's nice to have it in the user-friendly-er output too.