pantheon.switchboard-plug-onlineaccounts: use older vala

This is fixed upstream, but the patch doesn't apply cleanly, so simply
use vala-0.46 to work around the issue until next release.

https://github.com/elementary/switchboard-plug-onlineaccounts/pull/120

pkgs/desktops/pantheon/apps/switchboard-plugs/onlineaccounts/default.nix

@@ -5,7 +5,7 @@
 , meson
 , ninja
 , pkgconfig
-, vala
+, vala_0_46
 , libgee
 , granite
 , gtk3
@@ -39,7 +39,7 @@ stdenv.mkDerivation rec {
     meson
     ninja
     pkgconfig
-    vala
+    vala_0_46
   ];
 
   buildInputs = [
@@ -70,7 +70,7 @@ stdenv.mkDerivation rec {
 
   meta = with stdenv.lib; {
     description = "Switchboard Online Accounts Plug";
-    homepage = https://github.com/elementary/switchboard-plug-onlineaccounts;
+    homepage = "https://github.com/elementary/switchboard-plug-onlineaccounts";
     license = licenses.lgpl2Plus;
     platforms = platforms.linux;
     maintainers = pantheon.maintainers;