fix(elixir): fix credo detection for elixir linters. (#3809)

## What is this PR for?
Fix detection of elixir `credo` linter. 

`vim.fn.executable("credo") == 0` will never succeed because `credo` is
not binary/executable. It is a `mix` package and only available via `mix
credo` command.

Instead, the plugins(both `none-ls` and `nvim-lint`) will check for the
presence of the `.credo.exs` file.

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  why we should accept this pull request. -->

## Does this PR fix an existing issue?
Fixes #3808 
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  If this PR fixes any issues, please link to the issue here.
  Fixes #<issue_number>
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## Checklist
- [x] Both linters display credo warnings if `credo` is installed and
the `.credo.exs` config exists in a project.
- [x] There are no errors if the `.credo.exs` file does not exist in the
project.
- [x] I've read the
[CONTRIBUTING](https://github.com/LazyVim/LazyVim/blob/main/CONTRIBUTING.md)
guidelines.

lua/lazyvim/plugins/extras/lang/elixir.lua

@@ -33,12 +33,13 @@ return {
     "nvimtools/none-ls.nvim",
     optional = true,
     opts = function(_, opts)
-      if vim.fn.executable("credo") == 0 then
-        return
-      end
       local nls = require("null-ls")
       opts.sources = vim.list_extend(opts.sources or {}, {
-        nls.builtins.diagnostics.credo,
+        nls.builtins.diagnostics.credo.with({
+          condition = function(utils)
+            return utils.root_has_file(".credo.exs")
+          end,
+        }),
       })
     end,
   },
@@ -46,12 +47,17 @@ return {
     "mfussenegger/nvim-lint",
     optional = true,
     opts = function(_, opts)
-      if vim.fn.executable("credo") == 0 then
-        return
-      end
       opts.linters_by_ft = {
         elixir = { "credo" },
       }
+
+      opts.linters = {
+        credo = {
+          condition = function(ctx)
+            return vim.fs.find({ ".credo.exs" }, { path = ctx.filename, upward = true })[1]
+          end,
+        },
+      }
     end,
   },
 }