worktree: elucidate environment variables intended for child processes

worktree: elucidate environment variables intended for child processes

Take advantage of 'struct child_process.env' to make it obvious that
environment variables set by add_worktree() are intended specifically
for sub-commands it invokes to operate in the new worktree.

We assign a local 'struct argv_array' to child_process.env, rather than
utilizing the child_process.env_array 'struct argv_array', because
future patches will make add_worktree() invoke additional sub-commands,
and it's simpler to populate the environment array just once, whereas
child_process.env_array gets cleared after each invocation, thus would
require re-population for each sub-command.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>