Update error-parser for nix-instantiate on Nix 2.4+

In the (unreleased) Nix 2.4 the error-messages have been reformatted[1].
This patch aims to retain proper `.nix`-support in `ale`, for both
stable Nix (2.3 and older) and unstable Nix (2.4 and newer).

[1] https://github.com/NixOS/nix/pull/3590

test/handler/test_nix_handler.vader

@@ -5,6 +5,29 @@ After:
   call ale#linter#Reset()
 
 Execute(The nix handler should parse nix-instantiate error messages correctly):
+  AssertEqual
+  \ [
+  \   {
+  \     'lnum': 6,
+  \     'col': 3,
+  \     'type': 'E',
+  \     'text': "syntax error, unexpected ']', expecting ';'",
+  \   },
+  \   {
+  \     'lnum': 3,
+  \     'col': 5,
+  \     'type': 'E',
+  \     'text': "undefined variable 'foo'",
+  \   },
+  \
+  \ ],
+  \ ale_linters#nix#nix#Handle(bufnr(''), [
+  \   "@nix {\"line\":6,\"column\":3,\"raw_msg\":\"syntax error, unexpected ']', expecting ';'\"}",
+  \   "@nix {\"line\":3,\"column\":5,\"raw_msg\":\"undefined variable 'foo'\"}",
+  \   "@nix {\"unrelated\":\"message\"}"
+  \ ])
+
+Execute(The nix handler should parse message from old nix-instantiate correctly):
   AssertEqual
   \ [
   \   {