Fix potential buffer overflow in zpool command

The ZPOOL_SCRIPTS_PATH environment variable can be passed here. This
allows for arbitrarily long strings to be passed to sprintf(), which can
overflow the buffer.

I missed this in my earlier audit of the codebase. CodeQL's
cpp/unbounded-write check caught this.

Reviewed-by: Damian Szuberski <szuberskidamian@gmail.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14264