Harden build.sh script

[cbeauchesne]

Motivation

When ./build.sh -i runner was executed with an env var TEST_LIBRARY=cpp, it was trying to get the default weblog of cpp (which does not exists) and failed, even if the purpose of the call was not to build the weblog.

It was sneaky, as ./build.sh -i runner can be called by ./run.sh if the venv is updated. For cpp folks, it was breaking run.sh

Issue raised in #7082

Changes

Move weblog and variant resolution inside the block that actually build the weblog

Workflow

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Reviewer checklist

• Anything but tests/ or manifests/ is modified ? I have the approval from R&P team
• A docker base image is modified?
  • the relevant build-XXX-image label is present
• A scenario is added, removed or renamed?
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[github-actions]

CODEOWNERS have been resolved as:

utils/build/build.sh                                                    @DataDog/system-tests-core

[xlamorlette-datadog]

I confirm it's working fine.

[chatgpt-codex-connector]

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[datadog-datadog-prod-us1-2]

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

_{This comment will be updated automatically if new data arrives.
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[Copilot]

Pull request overview

This PR hardens utils/build/build.sh by deferring weblog validation and default-variant resolution until the script is actually building the weblog image, preventing failures when BUILD_IMAGES does not include weblog (e.g., ./build.sh -i runner with TEST_LIBRARY=cpp).

Changes:

• Move library existence checks, default weblog variant resolution, and variant Dockerfile validation into the weblog image build branch.
• Move remote Docker cache configuration to the weblog build path so it’s not computed when weblog isn’t being built.
• Remove the previous unconditional/global weblog validation block at the end of the script.

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