scripts/build_docs: don't assume GOPATH is set

[efd6]

What does this PR do?

This makes sure that the make docs target is able to find index.asciidoc files on systems where GOPATH has not been set.

Why is it important?

GOPATH is not a requirement for a functional go install, so having a failure in this case is a friction for contributors.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Author's Checklist

• [ ]

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[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @efd6? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[elasticmachine]

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• Start Time: 2022-11-28T21:25:31.079+0000

• Duration: 14 min 49 sec

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Test	Results
Failed	0
Passed	3
Skipped	0
Total	3

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