ci: Install ex

Author: alii

.github/workflows/test.yml

@@ -15,7 +15,7 @@ jobs:
           otp-version: '28'
           gleam-version: '1.13.0'
           rebar3-version: '3'
-          # elixir-version: "1"
+          elixir-version: '1.18.0'
       - run: gleam deps download
       - run: gleam test
       - run: gleam format --check src test