fix(siril): add python3 to the siril wrapper so it sees it in its runtime path

modules/nixos/astro.nix

@@ -11,8 +11,14 @@ let
     name = "siril-with-plugins";
     paths = [ pkgs.siril ];
     nativeBuildInputs = [ pkgs.makeWrapper ];
+    # Siril bootstraps its sirilpy venv at first run by locating a `python3`
+    # on PATH. nixpkgs only *propagates* python3 from the siril derivation (it
+    # isn't on the runtime PATH), so without the --prefix PATH below the GUI
+    # fails with "Python not found in system PATH" and never creates
+    # ~/.local/share/siril/venv. We add the same interpreter siril references.
     postBuild = ''
       wrapProgram $out/bin/siril \
+        --prefix PATH : ${lib.makeBinPath [ pkgs.python3 ]} \
         --prefix LD_LIBRARY_PATH : ${lib.makeLibraryPath (with pkgs; [
           stdenv.cc.cc.lib   # libstdc++ / libgcc_s / libgomp  (numpy etc.)
           zlib