Fix doc generation, remove cryptic references from plantuml

docs/dev/daemon-process-tree.plantuml

@@ -39,4 +39,3 @@ state AppDir {
 }
 
 @enduml
-

docs/shell.nix

@@ -1,14 +1,20 @@
 {
 
-  pkgs ? (import ../nix/pkgs.nix).pkgs,
+  buildSystem ? builtins.currentSystem,
+  hostSystem ? buildSystem,
+  pkgsNix ? (import ../nix/pkgs.nix),
 
-}: pkgs.mkShell {
+}: let
+  pkgs = pkgsNix.default {
+    inherit buildSystem hostSystem;
+  };
+in
+  pkgs.mkShell {
     name = "isle-build-docs";
     buildInputs = [ pkgs.plantuml ];
-
     shellHook = ''
       set -e
       plantuml -tsvg ./dev/*.plantuml
       exit 0
     '';
-}
+  }