configure CI to use a secret API key

[dshemetov]

todo: configure CI to use a key. Else I expect the examples to generate warning messages. Think we can use GitHub Secrets. I've played around with those here if you haven't already used them as part of production work.

Originally posted by @brookslogan in #79 (comment)

[dshemetov]

Currently our CI is building vignettes which make many calls to the API and this doesn't hit the API limit if not run too many times.

In the long run we should add a key as a secret.