fix(elixir mix): run formatter from project root (#576)

the current folder should be the project folder. There could be
per-subfolder formatter.exs files but you're not meant to run the
formatter from such folders.

Author: emmanueltouzery

lua/conform/formatters/mix.lua

@@ -7,7 +7,6 @@ return {
   command = "mix",
   args = { "format", "--stdin-filename", "$FILENAME", "-" },
   cwd = require("conform.util").root_file({
-    ".formatter.exs",
     "mix.exs",
   }),
 }