Add BUILDKITE_GCS_ACCESS_HOST for GCS Host choice

[jules2689]

Problem

• The URL for artifacts uploaded to GCS are api links.
• Following the link in browser yields an XML error page.
• The URL buildkite puts is under storage.googleapis.com but to leverage the oath prompt mentioned above users need to access objects under storage.cloud.google.com.
• I couldn't find anything from buildkite's doc equivalent to BUILDKITE_S3_ACCESS_URL for Google Cloud.

Alternatives

• When a user clicks a link in browser, you should use storage.cloud.google.com.
  • This flow is for humans, not machines - so it should be optimized for that experience
• When a user fetches a link via API, you should use storage.googleapis.com
  • This flow is for machines, not humans - so it should be optimized for that experience

This change would be simple but allow for us to easily leverage oAuth and API when needed.

Looking at our internal issue again, I think @mcgain has asked about this - so best to get his input 😄 I saw this pattern in S3 Uploader, so I copied.

cc @keithpitt @toolmantim

[mcgain]

In #528 I suggested BUILDKITE_GCS_ACCESS_URL to be consistent with BUILDKITE_S3_ACCESS_URL.

I agree that the alternatives are preferable, but I'm not sure whether they scale to other providers.

[jules2689]

Ah yea, I chose _HOST here to be consistent with the use of the env

[lox]

Sorry for the slow response!

This seems like a good incremental solution to me. We have similar issues with BUILDKITE_S3_ACCESS_URL where we need to come up with a way to serve that url, but signed for authentication. When we tackle that, we can figure out a way to improve the click to download experience for the GCS stuff too.