deskutils/pet: fix build on riscv64

[fuz]

Summary

deskutils/pet: fix build on riscv64

Bump golang.org/x/sys dependency to a version new enough to
have the bits required for riscv64-freebsd.

Sponsored by:	Berliner Linux User Group e.V.
Approved by:	portmgr (build fix blanket)
Approved by:	... (mentor)

Test Plan

Tested manually on riscv64 FreeBSD 13.1.
Tested with Poudriere on arm64 i386 amd64 FreeBSD 12.4 13.1.
See http://fuz.su/~fuz/freebsd/batch6

[yuri]

Thanks for the patch.
Please commit it.