glib: recognize NIX_GSETTINGS_OVERRIDES_DIR variable

For some reason, the GNOME 3.26 update broke the overrides. It turns
out the overrides now need to come before the overriden schemas in the
XDG_DATA_DIRS variable. This is not possible in general due to applications
prefixing the variable (e.g. in wrapGAppsHook).

To fix this, a new environment variable NIX_GSETTINGS_OVERRIDES_DIR
was introduced. It has greater priority than XDG_DATA_DIRS but lower
than GSETTINGS_SCHEMA_DIR. A separate variable was chosen in order not
to block the built-in one for users.

pkgs/development/libraries/glib/default.nix

@@ -55,7 +55,8 @@ stdenv.mkDerivation rec {
   };
 
   patches = optional stdenv.isDarwin ./darwin-compilation.patch
-    ++ optional doCheck ./skip-timer-test.patch;
+    ++ optional doCheck ./skip-timer-test.patch
+    ++ [ ./schema-override-variable.patch ];
 
   outputs = [ "out" "dev" "devdoc" ];
   outputBin = "dev";