numix-icon-theme-square: add missing parent icon themes

Also:
- add runHook calls
- move postFixup actions to installPhase

pkgs/data/icons/numix-icon-theme-square/default.nix

@@ -13,18 +13,16 @@ stdenv.mkDerivation rec {
 
   nativeBuildInputs = [ gtk3 ];
 
-  buildInputs = [ numix-icon-theme ];
-
-  propagatedBuildInputs = [ hicolor-icon-theme ];
+  propagatedBuildInputs = [ numix-icon-theme hicolor-icon-theme ];
 
   dontDropIconThemeCache = true;
 
   installPhase = ''
-    mkdir -p $out/share/icons
-    cp -a Numix-Square{,-Light} $out/share/icons/
-  '';
+    runHook preInstall
+
+    mkdir -p $out/share/icons
+    cp -a Numix-Square{,-Light} $out/share/icons
 
-  postFixup = ''
     for panel in $out/share/icons/*/*/panel; do
       ln -sf $(realpath ${numix-icon-theme}/share/icons/Numix/16/$(readlink $panel)) $panel
     done
@@ -32,6 +30,8 @@ stdenv.mkDerivation rec {
     for theme in $out/share/icons/*; do
       gtk-update-icon-cache $theme
     done
+
+    runHook postInstall
   '';
 
   meta = with stdenv.lib; {