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: yuri (maintainer)
Approved by: eduardo (mentor)
Differential Revision: https://reviews.freebsd.org/D38870