maintainers: add script and workflows to check sortedness

the script can output a list of sed commands to create the order it
expects to find. this was mainly useful for initially sorting the list,
but we'll keep it here for later reference.

Co-authored-by: Jörg Thalheim <Mic92@users.noreply.github.com>

.github/workflows/check-maintainers-sorted.yaml

@@ -0,0 +1,21 @@
+name: "Check that maintainer list is sorted"
+
+on:
+  pull_request:
+    paths:
+      - 'maintainers/maintainer-list.nix'
+permissions:
+  contents: read
+
+jobs:
+  nixos:
+    runs-on: ubuntu-latest
+    if: github.repository_owner == 'NixOS'
+    steps:
+      - uses: actions/checkout@v3
+      - uses: cachix/install-nix-action@v19
+        with:
+          # explicitly enable sandbox
+          extra_nix_config: sandbox = true
+      - name: Check that maintainer-list.nix is sorted
+        run: nix-instantiate --eval maintainers/scripts/check-maintainers-sorted.nix