Add systems input and rename 'utils' input to 'flake-utils'

[trueNAHO]

commit 428b3a54cabd0fd93fa487979e225c43de679785
Author: NAHO <90870942+trueNAHO@users.noreply.github.com>
Date:   2025-10-16 15:21:47 +0200

    Add systems input to leverage extensible flake systems pattern

    Add the systems input to transitively leverage the "externally
    extensible flake systems" [1] pattern.

    [1]: https://github.com/nix-systems/nix-systems

 flake.lock            | 5 ++++-
 flake.nix             | 8 +++++++-
 nix/tests/default.nix | 2 +-
 3 files changed, 12 insertions(+), 3 deletions(-)

commit 1686c4e1d45be322bdba161cc8bb3468e782107c
Author: NAHO <90870942+trueNAHO@users.noreply.github.com>
Date:   2025-10-16 15:31:49 +0200

    Conventionally rename 'utils' input to 'flake-utils'

 flake.lock            | 44 ++++++++++++++++++++++----------------------
 flake.nix             |  6 +++---
 nix/tests/default.nix |  6 +++---
 3 files changed, 28 insertions(+), 28 deletions(-)

commit 710f9d7c1a5a47af49a43581c716c657b98967db
Author: NAHO <90870942+trueNAHO@users.noreply.github.com>
Date:   2025-10-16 19:08:00 +0200

    Remove redundant aarch64-darwin system already present by default

    Remove the explicitly added aarch64-darwin system that is already
    included in the default systems input.

    Maybe this was not the case at the time of commit ed1ee1d86684 ("add
    aarch64-darwin to built systems") from four years ago.

    Reverts: ed1ee1d86684 ("add aarch64-darwin to built systems")

 flake.nix | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

The first two patches are strongly inspired by NotAShelf/nvf#1001, while the last one is merely a cleanup.

I tested this PR with nix flake check in this repo and by running nix flake check in my NixOS configuration.