remove flake-utils from the flake inputs

As requested in <https://todo.sr.ht/~soywod/pimalaya/131> I removed
flake-utils. This reduces the number of flake inputs and doesn't add
much code.

The way this works, is that instead of `eachDefaultSystem` we have a
function `forEachSupportedSystem`, this function generates an attrset
with a key for each system in the `supportedSystems` array, whose value
is the result of calling the provided function with the system as an
argument:

```nix repl
repl> forEachSupportedSystem f
{
  "x86_64-linux" = f "x86_64-linux";
  ...
}
```

This is slightly clumsier than `flake-utils.lib.eachDefaultSystem`,
which rewrites the returned attrset, but it is much less code and
simpler to understand.

I tested the build with `nix build` on `x86_64-linux` and it still works
c:

flake.lock

@@ -37,24 +37,6 @@
         "type": "github"
       }
     },
-    "flake-utils": {
-      "inputs": {
-        "systems": "systems"
-      },
-      "locked": {
-        "lastModified": 1705309234,
-        "narHash": "sha256-uNRRNRKmJyCRC/8y1RqBkqWBLM034y4qN7EprSdmgyA=",
-        "owner": "numtide",
-        "repo": "flake-utils",
-        "rev": "1ef2e671c3b0c19053962c07dbda38332dcebf26",
-        "type": "github"
-      },
-      "original": {
-        "owner": "numtide",
-        "repo": "flake-utils",
-        "type": "github"
-      }
-    },
     "gitignore": {
       "inputs": {
         "nixpkgs": [
@@ -115,7 +97,6 @@
       "inputs": {
         "fenix": "fenix",
         "flake-compat": "flake-compat",
-        "flake-utils": "flake-utils",
         "gitignore": "gitignore",
         "naersk": "naersk",
         "nixpkgs": "nixpkgs"
@@ -137,21 +118,6 @@
         "repo": "rust-analyzer",
         "type": "github"
       }
-    },
-    "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",