CI: Bump lint stage to elixir-1.12

Elixir 1.12 changed formatting rules, this allows to avoid having to rollback to run `mix format`

test/support/api_spec_helpers.ex

@@ -29,9 +29,7 @@ defmodule Pleroma.Tests.ApiSpecHelpers do
           end)
 
         flunk(
-          "Value does not conform to schema #{schema.title}: #{Enum.join(errors, "\n")}\n#{
-            inspect(value)
-          }"
+          "Value does not conform to schema #{schema.title}: #{Enum.join(errors, "\n")}\n#{inspect(value)}"
         )
     end
   end