ci: docs changeset against the previous commit

[v1v]

What does this PR do?

Override the default behaviour when comparing the current change set with the previous one.

Avoid aborting any ongoing builds if a new commit is pushed and it does only contain docs changes.

Why

Give a higher precedence to the GIT_PREVIOUS_COMMIT env variable, then if there are a subsequent commits within the same Pull Request, it will compare the both commits rather than the whole changeset between the PR and the main.

Ideally, this should help with running faster builds by detecting what to run smarter.

[apmmachine]

:green_heart: Build Succeeded

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Build stats

• Start Time: 2022-11-02T11:39:26.090+0000

• Duration: 38 min 33 sec

Test stats :test_tube:

Test	Results
Failed	0
Passed	2516
Skipped	31
Total	2547

:green_heart: Flaky test report

Tests succeeded.

:robot: GitHub comments

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To re-run your PR in the CI, just comment with:

• /test : Re-trigger the build.

• run benchmark tests : Run the benchmark tests.

• run jdk compatibility tests : Run the JDK Compatibility tests.

• run integration tests : Run the Agent Integration tests.

• run end-to-end tests : Run the APM-ITs.

• run windows tests : Build & tests on windows.

• run elasticsearch-ci/docs : Re-trigger the docs validation. (use unformatted text in the comment!)

[github-actions[bot]]

/test

[v1v]

I've just realised the from transformation is not working as expected, so I'm debugging what's going on, sorry for the spam!

[v1v]

I'll close this for now as we are evaluating other CI options in the mid term, for now, we won't put much effort on this for avoiding the corner cases if no detecting changes when needed correctly