Use compile env to run install mix task

This ensures that if the :auto_install configuration changes,
the module will recompile, and the hooks will be installed or
skipped accordingly.

lib/git_hooks.ex

@@ -3,7 +3,9 @@ defmodule GitHooks do
 
   alias Mix.Tasks.GitHooks.Install
 
-  if Application.get_env(:git_hooks, :auto_install, true) do
+  @auto_install Application.compile_env(:git_hooks, :auto_install, true)
+
+  if @auto_install do
     Install.run(["--quiet"])
   end