Port away from flake-utils (#35675)

`flake-utils` is currently only used for outputting system-specific dev
shells. This can actually be achieved only using functionality already
present within `nixpkgs`, thus there is no need for an extra dependency.

Additionally, we move to use the `packages` and `env` args for `mkShell`
to more clearly outline what they are used for.

---

Further reading:
https://determinate.systems/blog/best-practices-for-nix-at-work/#avoid-flake-helper-libraries-if-possible

As a side note, using `with` to import large scopes is [discouraged by
official Nix
resources](https://nix.dev/guides/best-practices#with-scopes), so an
alternative approach to list installed packages could be something like
this:

```nix
packages =
  (builtins.attrValues {
    inherit (pkgs)
      # generic
      git
      git-lfs
      gnumake
      gnused
      gnutar
      gzip
      zip

      # frontend
      cairo
      pixman
      pkg-config

      # linting
      uv

      # backend
      gofumpt
      sqlite
      ;

    inherit
      # frontend
      nodejs
      pnpm

      # linting
      python3

      # backend
      go
      ;
  })
  ++ linuxOnlyInputs;
```

But I saw this as too pedantic to include in the initial PR.

Co-authored-by: 6543 <6543@obermui.de>

flake.lock

@@ -1,23 +1,5 @@
 {
   "nodes": {
-    "flake-utils": {
-      "inputs": {
-        "systems": "systems"
-      },
-      "locked": {
-        "lastModified": 1731533236,
-        "narHash": "sha256-l0KFg5HjrsfsO/JpG+r7fRrqm12kzFHyUHqHCVpMMbI=",
-        "owner": "numtide",
-        "repo": "flake-utils",
-        "rev": "11707dc2f618dd54ca8739b309ec4fc024de578b",
-        "type": "github"
-      },
-      "original": {
-        "owner": "numtide",
-        "repo": "flake-utils",
-        "type": "github"
-      }
-    },
     "nixpkgs": {
       "locked": {
         "lastModified": 1760038930,
@@ -36,24 +18,8 @@
     },
     "root": {
       "inputs": {
-        "flake-utils": "flake-utils",
         "nixpkgs": "nixpkgs"
       }
-    },
-    "systems": {
-      "locked": {
-        "lastModified": 1681028828,
-        "narHash": "sha256-Vy1rq5AaRuLzOxct8nz4T6wlgyUR7zLU309k9mBC768=",
-        "owner": "nix-systems",
-        "repo": "default",
-        "rev": "da67096a3b9bf56a91d16901293e51ba5b49a27e",
-        "type": "github"
-      },
-      "original": {
-        "owner": "nix-systems",
-        "repo": "default",
-        "type": "github"
-      }
     }
   },
   "root": "root",