get_super_prefix(): copy getenv() result

get_super_prefix(): copy getenv() result

The return value of getenv() is not guaranteed to remain valid across
multiple calls (nor across calls to setenv()). Since this function
caches the result for the length of the program, we must make a copy to
ensure that it is still valid when we need it.

Reported-by: Yngve N. Pettersen <yngve@vivaldi.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>