Fv3net CI integration is currently using a "user" key

[frodre]

I'm not sure how expansive this repositories CI workflow access needs to be at present.

Previously, before needing to rotate keys, access was granted via a user key (from @mcgibbon's account, I believe). I switched the user key to my account temporarily. Ideally, this should just be a deploy key for just fv3net access, or a machine user if (@climate-ci-github) if we need access to more than one private repo. Also should remove the extra ssh fingerprints left in the CI config.

[mcgibbon]

Going to echo that this should be swapped to the climate-ci-github user key if it needs to be a user key.