CI: don't decrypt the deploy key unless we're trying to deploy
CI: don't decrypt the deploy key unless we're trying to deploy

PRs don't have access to the deploy key, for security reasons, so this
shouldn't be in the general steps

greatscottgadgets/greatfet,greatscottgadgets/greatfet,dominicgs/GreatFET-experimental,greatscottgadgets/greatfet,greatscottgadgets/greatfet,dominicgs/GreatFET-experimental,dominicgs/GreatFET-experimental