[CI] Sign files using gpg --yes

[christophermaier]

We've seen intermittent failures to upload manifest.json files
because when we go to sign it, gpg says there's already a
manifest.json.asc file present, and asks whether we want to
overwrite it or not. As it's a tiny bit tricky to get STDIN in a
Buildkite stage, the stage ends up timing out.

Here, we simply add the --yes option, to automatically answer such
questions in the affirmative.

I'm not exactly sure how a manifest.json.asc could be present in a
clean runner, though.

Signed-off-by: Christopher Maier cmaier@chef.io