test(tvix): nix-planned test verification using C++ Nix 2.3 and 2.11

Reimplement the test discovery of the lang tests script in Nix which
allows for a more flexible skipping logic that can e.g. react to the C++
Nix version used. This allows us to run the test suite against both
C++ Nix 2.3 and the latest C++ Nix version 2.11. The latter is mainly
useful, so we can implement newer Nix features and still verify them
against the C++ implementation.

Change-Id: I30c802844133b86b5e49f5e4f4fefacdb6215e0e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7042
Autosubmit: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

third_party/overlays/tvl.nix

@@ -26,6 +26,8 @@ in
     systems = [ builtins.currentSystem ];
   }).build."${builtins.currentSystem}";
 
+  nix_latest = super.nix;
+
   clang-tools_11 = self.clang-tools.override {
     llvmPackages = self.llvmPackages_11;
   };