chore: rebrand Nix to Lix when it makes sense

Here's my guide so far:

$ rg '((?!(recursive).*) Nix
(?!(daemon|store|expression|Rocks!|Packages|language|derivation|archive|account|user|sandbox|flake).*))'
-g '!doc/' --pcre2

All items from this query have been tackled. For the documentation side:
that's for https://git.lix.systems/lix-project/lix/issues/162.

Additionally, all remaining references to github.com/NixOS/nix which
were not relevant were also replaced.

Fixes: https://git.lix.systems/lix-project/lix/issues/148.
Fixes: https://git.lix.systems/lix-project/lix/issues/162.
Change-Id: Ib3451fae5cb8ab8cd9ac9e4e4551284ee6794545
Signed-off-by: Raito Bezarius <raito@lix.systems>

tests/functional/repl_characterization/test-session.cc

@@ -173,7 +173,7 @@ void TestSession::runCommand(std::string command)
         std::cerr << "runCommand " << command << "\n";
     }
     command += "\n";
-    // We have to feed a newline into the output parser, since Nix might not
+    // We have to feed a newline into the output parser, since Lix might not
     // give us a newline before a prompt in all cases (it might clear line
     // first, e.g.)
     outputParser.feed('\n');