nix flakes metadata: Show lastModified timestamp for each input

[wentasah]

Motivation

Before this commit, the output of nix flake metadata in nix repo looked like this:

...
Last modified: 2023-07-09 16:00:16
Inputs:
├───flake-compat: github:edolstra/flake-compat/35bb57c0c8d8b62bbfd284272c928ceb64ddbde9
├───lowdown-src: github:kristapsdz/lowdown/d2c2b44ff6c27b936ec27358a2653caaef8f73b8
├───nixpkgs: github:NixOS/nixpkgs/04a75b2eecc0acf6239acf9dd04485ff8d14f425
└───nixpkgs-regression: github:NixOS/nixpkgs/215d4d0fd80ca5163643b03a33fde804a29cc1e2

This commit changes that to:

...
Last modified: 2023-07-09 16:00:16
Inputs:
├───flake-compat: github:edolstra/flake-compat/35bb57c0c8d8b62bbfd284272c928ceb64ddbde9 (2023-01-17 11:47:33)
├───lowdown-src: github:kristapsdz/lowdown/d2c2b44ff6c27b936ec27358a2653caaef8f73b8 (2021-10-06 10:00:07)
├───nixpkgs: github:NixOS/nixpkgs/04a75b2eecc0acf6239acf9dd04485ff8d14f425 (2022-12-08 01:04:00)
└───nixpkgs-regression: github:NixOS/nixpkgs/215d4d0fd80ca5163643b03a33fde804a29cc1e2 (2022-01-24 19:20:45)

This makes it easier to figure out how old the inputs are.

Context

Originally proposed in #7729. Its author did not implement requested changes. This PR implements them.

Closes #7729.

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

Add 👍 to pull requests you find important.