docs: do not rebuild on each change

this does slight tweaking of paths to make the docs.hacc.space
derivation no longer depend on our entire flake, so we won't have to
rebuild it as often.

flake.nix

@@ -58,9 +58,9 @@
         system = "x86_64-linux";
         modules = [
           ./parsons/configuration.nix
+          ./modules/buildinfo.nix
           sops-nix.nixosModules.sops
           { nixpkgs.pkgs = pkgs; }
-          { environment.etc."haccfiles".source = self.outPath; }
         ];
         specialArgs = {
           sources = inputs;

modules/buildinfo.nix

@@ -0,0 +1,30 @@
+{ config, lib, pkgs, sources, ... }:
+
+let
+  self = sources.self;
+  slice = string: start: length: with lib;
+    strings.concatStrings
+      (lists.take length
+        (lists.drop start
+          (strings.stringToCharacters string)));
+
+  formatDate = date:
+    let
+      year = slice date 0 4;
+      month = slice date 4 2;
+      day = slice date 6 2;
+      hour = slice date 8 2;
+      minute = slice date 10 2;
+      second = slice date 12 2;
+    in
+      "${year}-${month}-${day} ${hour}:${minute}:${second} UTC";
+in
+{
+  system.nixos.label = "${config.system.nixos.release}-haccfiles-${self.shortRev or self.dirtyShortRev}";
+  users.motd = ''
+    Welcome to ${config.networking.hostName}, running NixOS ${config.system.nixos.release}!
+    Built from haccfiles ${self.rev or self.dirtyRev}.
+    Last commit was at ${formatDate self.lastModifiedDate}.
+    ${if self ? dirtyRev then "\nPlease remember to commit your changes.\n" else ""}
+  '';
+}

websites/docs.hacc.space/default.nix

@@ -1,23 +1,17 @@
-{ stdenvNoCC, zola, writeScriptBin }:
+{ copyPathToStore, stdenvNoCC, zola, writeScriptBin }:
 
 stdenvNoCC.mkDerivation rec {
   name = "docs.hacc.space-static";
 
-  # HINT: this is cursed. Nix flakes have no optimisation to deal with ${./.},
-  # so we wind up having to do this to make the symlink to content/ work.
-  # (we still need to manually adjust it — but at least this way we can find
-  # its target without further hoops)
-  #
-  # This does also mean we now copy the entire flake into the Nix store twice.
-  # Yay for flakes!
-  src = "${../../.}/websites/docs.hacc.space";
+  src = ./.;
+  content = copyPathToStore ../../docs;
 
   phases = [ "buildPhase" ];
   buildInputs = [ zola ];
   buildPhase = ''
     cp -r $src/* .
     rm content
-    ln -s $src/../../docs content
+    ln -s $content content
     zola build --output-dir $out
   '';