Use format_hex_number_sigsafe to format syscalls in sandbox.c

Use format_hex_number_sigsafe to format syscalls in sandbox.c

This way, we don't have to use snprintf, which is not guaranteed to
be signal-safe.

(Technically speaking, strlen() and strlcpy() are not guaranteed to
be signal-safe by the POSIX standard. But I claim that they are on
every platform that supports libseccomp2, which is what matters
here.)