envars: skip Prepend when value is already at front of list

[stuartwdouglas]

When Prepend.Apply() is called and the value's path components are already a prefix of the current list, skip the operation entirely. This prevents hermit exec from reordering PATH entries that were intentionally placed before Hermit-managed paths by the caller.

Previously, hermit exec unconditionally removed and re-prepended all Hermit-managed paths, which pushed non-Hermit entries (like wrapper script directories) behind Hermit paths even when the caller had already set up the correct PATH order.

The hermit_on_activate shell hook exists to fix this reordering, but it only runs in interactive shells — not during hermit exec via bin stubs.