feat(agent-go): skip build for if there are no code changes

[kruskall]

try to run the experiment with apm-agent-go for now
only run the diff command for apm-agent-go
fetch the target repo to avoid errors on repo different from main

apm-agent-go is in maintenance only mode so this should have no impact on a supported product.

I'll run some tests on different branches with different names to ensure everything is working and then we can do a gradual rollout to other repositories

[github-actions]

A documentation preview will be available soon.

• 🔨 Buildkite builds
• 📚 HTML diff
• 📙 Preview page

Request a new doc build by commenting

• Rebuild this PR: run docs-build
• Rebuild this PR and all Elastic docs: run docs-build rebuild

_{run docs-build is much faster than run docs-build rebuild. A rebuild should only be needed in rare situations.}

_{If your PR continues to fail for an unknown reason, the doc build pipeline may be broken. Elastic employees can check the pipeline status here.}