libxmlb: fix installedTest

nixos/tests/installed-tests/libxmlb.nix

···
 
 makeInstalledTest {
   tested = pkgs.libxmlb;
+
+  testConfig = {
+    environment.variables = {
+      G_TEST_SRCDIR = "${pkgs.libxmlb.installedTests}/libexec/installed-tests/libxmlb";
+    };
+  };
 }