security/nebula: fix build on riscv64

Bump golang.org/x/{net,term,sys} dependencies to a version
recent enough to support a build on riscv64-freebsd.

Sponsored by:	Berliner Linux User Group e.V.
Approved by:	portmgr (build fix blanket)
Differential Revision: https://reviews.freebsd.org/D39162