nixosTests.installer.*: disable documentation

building the manual takes a pretty long time and sufficient memory to
cause hydra failures: https://hydra.nixos.org/build/205689097#tabs-buildsteps

nixos/tests/installer.nix

@@ -21,6 +21,8 @@ let
             <nixpkgs/nixos/modules/testing/test-instrumentation.nix>
           ];
 
+        documentation.enable = false;
+
         # To ensure that we can rebuild the grub configuration on the nixos-rebuild
         system.extraDependencies = with pkgs; [ stdenvNoCC ];