This bump nixpkgs input to 25.11 and fix the two warnings that were
coming with the update:
1. `pkgs.system` => `pkgs.stdenv.hostPlatform.system`
2. `runCommandNoCC` => `runCommand`
also:
1. remove cppnix 2.3 compat tests (cppnix 2.3 is officially dead now)
2. remove lowdown 1.3 compat tests (nixpkgs no longer carries it)
Co-authored-by: Tom Hubrecht <github@mail.hubrecht.ovh>
Change-Id: I78526b5d8992a6c63ecd7f0c7c1fa6346a6a6964