Set build type for scheduled builds based on doc repo changes

[bmorelli25]

Summary

In #2929, we reverted to incremental builds run every 30 minutes. This works great, until a PR is merged to the elastic/docs repository. Almost every PR merged in this repo requires a rebuild in order for the changes to take effect on elastic.co/guide/*.

This PR updates our scheduled builds to first check to see if the build commit of elastic/docs has changed since the previous successful, scheduled run. If the commit has changed, we run a full docs rebuild to ensure the elastic/docs changes are propagated to all corners of elastic.co/guide. If the commit hasn't changed, there's no need for a full rebuild, and we instead run an incremental build—the faster and cheaper option.

Testing

Testing this PR is fun and easy!

• You can run the bash script locally.
• You can kick off a BK build here: https://buildkite.com/elastic/docs-build/builds?branch=test-conditional-builds. Note, you must use the penultimate commit: c3e8250. Here's the successful run: https://buildkite.com/elastic/docs-build/builds/3589#018d8ce5-6265-4b9d-a8ff-49e0b8c4612a.

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[bmorelli25]

@nkammah why does L30 not need bash in the command but L35 does?

[nkammah]

We specify the script interpreter in the first line of the script here - that's what we call the "shebang". If the script is executable (ex. running "chmod a+x" on it), the pipeline will know how to call it.

As for the discrepancy on L35, it could be legacy, or that we explicitly specified it at the time. Both approaches will work.