Previously, the `pie install` would invoke another instance of `pie`. However,
since this would always be run in a non-interactive shell, `sudo` prompts will
never work. I've changed this instead to use `InvokeSubCommand` which copies
the input params into the sub command, so it is all run in the same PIE process
and therefore retains the interactivity (or lack thereof) of the shell it was
invoked from.
