Indicate when in Homesick shell

Set env var after `homesick cd`. This allows custom prompts to add a
visual indicator that are in a shell within a shell.

Author: jakeprime

lib/homesick/cli.rb

@@ -237,7 +237,8 @@ def cd(castle = DEFAULT_CASTLE_NAME)
                  "Opening a new shell in castle '#{castle}'. To return to the original one exit from the new shell.",
                  :green
       inside castle_dir do
-        system(ENV.fetch('SHELL', nil))
+        shell = ENV.fetch('SHELL', nil)
+        system("HOMESICK_SHELL=1 #{shell}") if shell
       end
     end