pantheon: more build fixes by using older vala

An extension of commit b49e798.  /cc PR #92592.
I expect it's "safer" when whole pantheon uses the same version.
Fixes build of wingpanel-indicator-session; maybe some others, too.
Now I built whole pantheon attrset and tests.pantheon.x86_64-linux

pkgs/desktops/pantheon/default.nix

@@ -87,7 +87,6 @@ lib.makeScope pkgs.newScope (self: with self; {
 
   gala = callPackage ./desktop/gala {
     inherit (gnome3) gnome-desktop;
-    vala = pkgs.vala_0_46;
   };
 
   wingpanel = callPackage ./desktop/wingpanel { };
@@ -199,12 +198,12 @@ lib.makeScope pkgs.newScope (self: with self; {
 
   elementary-wallpapers = callPackage ./artwork/elementary-wallpapers { };
 
+  vala = pkgs.vala_0_46; # multiple pantheon packages have issues with vala 0.48.7
+
 } // lib.optionalAttrs (config.allowAliases or true) {
 
   ### ALIASES
 
-  inherit (pkgs) vala; # added 2019-10-10
-
   cerbere = throw "Cerbere is now obsolete https://github.com/elementary/cerbere/releases/tag/2.5.1.";
 
 })