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>

src/libmain/common-args.cc

@@ -33,7 +33,7 @@ MixCommonArgs::MixCommonArgs(const std::string & programName)
 
     addFlag({
         .longName = "option",
-        .description = "Set the Nix configuration setting *name* to *value* (overriding `nix.conf`).",
+        .description = "Set the Lix configuration setting *name* to *value* (overriding `nix.conf`).",
         .category = miscCategory,
         .labels = {"name", "value"},
         .handler = {[this](std::string name, std::string value) {