Update golang.org/x/sys to fix build issue with Go 1.18

[folbricht]

Fixes a failing build on darwin

$ GOOS=darwin go build
# golang.org/x/sys/unix
../../../../go/pkg/mod/golang.org/x/sys@v0.0.0-20201201145000-ef89a241ccb3/unix/syscall_darwin.1_13.go:29:3: //go:linkname must refer to declared function or variable
../../../../go/pkg/mod/golang.org/x/sys@v0.0.0-20201201145000-ef89a241ccb3/unix/zsyscall_darwin_amd64.1_13.go:27:3: //go:linkname must refer to declared function or variable
../../../../go/pkg/mod/golang.org/x/sys@v0.0.0-20201201145000-ef89a241ccb3/unix/zsyscall_darwin_amd64.1_13.go:40:3: //go:linkname must refer to declared function or variable