Update golang.org/x/sys to fix macOS builds

[mjlshen]

Fixes #433

Build completes successfully afterwards:

❯ make build
go get
go mod verify
all modules verified
go mod vendor
go build -mod=vendor -o pd ./command

❯ ./pd
Usage: pd [--version] [--help] <command> [<args>]

Available commands are:
    ability
    addon
    analytics
    escalation-policy
    event-v2
    incident
    log-entry
    maintenance-window
    notification
    oncall
    schedule
    service
    team
    user
    vendor

[ChezCrawford]

Thanks for taking care of this @mjlshen !