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/fetchTree-file.sh

@@ -72,7 +72,7 @@ EOF
     # old 'tarball' fetcher should still have their type set to 'tarball'
     assert (nodes.tarball_default_unpack.locked.type == "tarball");
     # Unless explicitely specified, the 'unpack' parameter shouldn’t appear here
-    # because that would break older Nix versions
+    # because that would break older Nix implementation versions
     assert (!nodes.tarball_default_unpack.locked ? unpack);
     assert (nodes.tarball_default_unpack.locked.narHash == "$input_directory_hash");
 
@@ -94,7 +94,7 @@ EOF
     [[ -z "${NIX_DAEMON_PACKAGE-}" ]] && return 0
 
     # Ensure that a lockfile generated by the current Nix for tarball inputs
-    # can still be read by an older Nix
+    # can still be read by an older Nix implementation
 
     cat <<EOF > flake.nix
     {