meld: avoid -dev paths in runtime closure

Use `pythonPath` instead of `propagatedBuildInputs`.

Closure before the change:

$ nix path-info -rsSh $(nix-build '<nixpkgs>' -A meld) | unnix | nl | tail -n1
185 /<<NIX>>/meld-3.22.0 5.2M 512.7M

After the change:

$ nix path-info -rsSh $(nix-build -A meld) | unnix | nl | tail -n1
155 /<<NIX>>/meld-3.22.0 5.2M 480.8M

This way we avoid 30 depends and 32MB of closure. Those were the
following packages:

/<<NIX>>/brotli-1.1.0
/<<NIX>>/brotli-1.1.0-dev
/<<NIX>>/bzip2-1.0.8-dev
/<<NIX>>/cairo-1.16.0-dev
/<<NIX>>/expat-2.5.0-dev
/<<NIX>>/fontconfig-2.14.2-bin
/<<NIX>>/fontconfig-2.14.2-dev
/<<NIX>>/freetype-2.13.2-dev
/<<NIX>>/gettext-0.21.1
/<<NIX>>/glib-2.76.4-bin
/<<NIX>>/glib-2.76.4-dev
/<<NIX>>/glibc-2.38-23-bin
/<<NIX>>/glibc-2.38-23-dev
/<<NIX>>/glibc-iconv-2.38
/<<NIX>>/libelf-0.8.13
/<<NIX>>/libffi-3.4.4-dev
/<<NIX>>/libGL-1.6.0-dev
/<<NIX>>/libglvnd-1.6.0-dev
/<<NIX>>/libpng-apng-1.6.40-dev
/<<NIX>>/libX11-1.8.7-dev
/<<NIX>>/libXau-1.0.11-dev
/<<NIX>>/libxcb-1.16-dev
/<<NIX>>/libXext-1.3.5-dev
/<<NIX>>/libXrender-0.9.11-dev
/<<NIX>>/linux-headers-6.5
/<<NIX>>/python3.11-pygobject-3.44.1-dev
/<<NIX>>/xcb-util-0.4.1
/<<NIX>>/xcb-util-0.4.1-dev
/<<NIX>>/xorgproto-2023.2
/<<NIX>>/zlib-1.3-dev

pkgs/applications/version-management/meld/default.nix

···
     gnome.adwaita-icon-theme
   ];
 
-  propagatedBuildInputs = with python3.pkgs; [
+  pythonPath = with python3.pkgs; [
     pygobject3
     pycairo
   ];