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 {
-  name = "isle-build-docs";
-  buildInputs = [ pkgs.plantuml ];
-
-  shellHook = ''
-    set -e
-    plantuml -tsvg ./dev/*.plantuml
-    exit 0
-  '';
-}
+}: 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
+    '';
+  }