Remove Leaked indirect Input

[benj2468]

Removes the leaked indirect inputs.systems. This created an ambiguous and indirect lockfile entry, this is resolved with this PR, which creates a clear original entry with fields similar to the locked entry.

Furthermore, not having it marked as an input is not inline with the examples of the nix-systems project found here.

Contribution checklist (recommended but not always applicable/required):

• There's an automated test for this change
• Commit messages or code include references to related issues or PRs (including third parties)
• Commit messages are conventional - examples from the log include "feat: add changelog files to fixup hook", "fix(contourpy): allow wheel usage", and "test: add sqlalchemy2 test"

[elikoga]

Good catch!

This means that systems will not get resolved via https://github.com/NixOS/flake-registry but directly. And since we actually depend on it, a explicit reference in the inputs is good.