use env_array member of struct child_process

use env_array member of struct child_process

Convert users of struct child_process to using the managed env_array for
specifying environment variables instead of supplying an array on the
stack or bringing their own argv_array.  This shortens and simplifies
the code and ensures automatically that the allocated memory is freed
after use.

Signed-off-by: Rene Scharfe <redacted>
Signed-off-by: Junio C Hamano <redacted>