gnvim: fix build

When running the default builder for Rust, the artifacts would be stored
in `target/<arch>/<profile>`, however the `install`-target expects the
default structure (`target/<profile>`) of `cargo`-builds.

When using the Makefile for building as well, the expected structure is
created instead.

pkgs/applications/editors/neovim/gnvim/default.nix

@@ -33,6 +33,10 @@ rustPlatform.buildRustPackage rec {
     EOF
   '';
 
+  buildPhase = ''
+    make build
+  '';
+
   installPhase = ''
     make install PREFIX="${placeholder "out"}"
   '';