workflows/keep-sorted: drop and move to treefmt

Same reasoning as the commit before, but keep-sorted has even less
overhead than editorconfig-checker. Benchmark has it at 1 second per
run.

.github/workflows/keep-sorted.yml

@@ -1,41 +0,0 @@
-name: Check that files are sorted
-
-on:
-  pull_request_target:
-    types: [opened, synchronize, reopened]
-
-permissions: {}
-
-jobs:
-  get-merge-commit:
-    uses: ./.github/workflows/get-merge-commit.yml
-
-  nixos:
-    name: keep-sorted
-    runs-on: ubuntu-24.04
-    needs: get-merge-commit
-    if: "needs.get-merge-commit.outputs.mergedSha && !contains(github.event.pull_request.title, '[skip treewide]')"
-    steps:
-      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
-        with:
-          ref: ${{ needs.get-merge-commit.outputs.mergedSha }}
-
-      - name: Get Nixpkgs revision for keep-sorted
-        run: |
-          # Pin to a commit from nixpkgs-unstable to avoid e.g. building nixfmt from staging.
-          # This should not be a URL, because it would allow PRs to run arbitrary code in CI!
-          rev=$(jq -r .rev ci/pinned-nixpkgs.json)
-          echo "url=https://github.com/NixOS/nixpkgs/archive/$rev.tar.gz" >> "$GITHUB_ENV"
-
-      - uses: cachix/install-nix-action@526118121621777ccd86f79b04685a9319637641 # v31
-        with:
-          extra_nix_config: sandbox = true
-          nix_path: nixpkgs=${{ env.url }}
-
-      - name: Install keep-sorted
-        run: "nix-env -f '<nixpkgs>' -iAP keep-sorted jq"
-
-      - name: Check that Nix files are sorted
-        shell: bash
-        run: |
-          git ls-files | xargs keep-sorted --mode lint | jq --raw-output '.[] | "Please make sure any new entries in \(.path) are sorted alphabetically."'

ci/default.nix

@@ -44,6 +44,8 @@ let
         # By default it's info, which is too noisy since we have many unmatched files
         settings.on-unmatched = "debug";
 
+        programs.keep-sorted.enable = true;
+
         # This uses nixfmt-rfc-style underneath,
         # the default formatter for Nix code.
         # See https://github.com/NixOS/nixfmt