Resolve BUILDKITE_COMMIT before pipeline upload

[lox]

This resolves BUILDKITE_COMMIT (which can be HEAD or FETCH_HEAD) to an actual commit before interpolation into a pipeline that is being uploaded.

Closes #456.

[toolmantim]

Oh my, this removes so much many hacks! 👏🏼

[lox]

For historical context, this would be better placed at the bootstrap level so that anything in the job gets the resolved value for $BUILDKITE_COMMIT, but our concern was that might be too much of a breaking change. This hacky fix at least prevents people running into the issue and we can re-evaluate when we move to 4.0.0.

[nullobject]

Ace, thank you. What a lovely xmas present 🎅❤️

[keith]

It seems like this case may still exist if you're using a scheduled job that sets HEAD as the commit? In that case I still see BUILDKITE_COMMIT being set as HEAD