Replace the HOMEBREW_TAP_TOKEN PAT with HOMEBREW_TAP_DEPLOY_KEY, a
write-enabled SSH deploy key scoped to only AdrianBonpin/homebrew-gridline.
actions/checkout's `ssh-key` input wires the key into the SSH agent for the
job, so the cask push goes over SSH and no broadly-scoped personal access
token has to be stored as a repository secret.
Also drops the now-wrong claim that the job no-ops until the secret is set.