transport-helper: drop const to fix strchr() warnings

transport-helper: drop const to fix strchr() warnings

We implicitly drop the const from our "key" variable when we do:

  char *p = strchr(key, ' ');

which causes compilation with some C23 versions of libc (notably recent
glibc) to complain.

We need "p" to remain writable, since we assign NUL over the space we
found. We can solve this by also making "key" writable. This works
because it comes from a strbuf, which is itself a writable string.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>