selftests/nolibc: use mutable buffer for execve() argv string

selftests/nolibc: use mutable buffer for execve() argv string

The existing code would trigger a warning under -Wwrite-strings which is
about to be enabled. Use a mutable buffer instead. While in this
specific case, casting away the 'const' would be fine, let's avoid casts
which are not really necessary.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://patch.msgid.link/20260525-nolibc-write-strings-v2-2-ab5cc16c7b23@weissschuh.net