fix: support installs without yarn.lock

[pabloerhard]

Summary

• Allow the Node 18/20 dependency installer to run when its input tree does not contain yarn.lock.
• Preserve and restore an existing lockfile, while removing a lockfile generated during installation when the input did not include one.
• Add a regression test for the lockfile-free build context and run it in CI.

Failure

After #818 changed the layer Dockerfile to call scripts/install_deps.sh, the DataDog/dd-trace-js default GitLab pipeline began failing while building the Node 20 Lambda layer for dd-trace-js PR #9812:

Node 20 is not supported by dd-trace v6, pinning dd-trace to 5.126.0
cp: cannot stat 'yarn.lock': No such file or directory

The downstream pipeline uses a reduced build context that contains package.json and the installer scripts but not the repository lockfile. Yarn can install and generate a temporary lockfile in that context; only the installer's unconditional backup made the lockfile mandatory.

Testing

• bash -n scripts/install_deps.sh scripts/test_install_deps.sh
• ./scripts/test_install_deps.sh

Authored by: @crysmags

[datadog-datadog-prod-us1]

⚠️ Warnings

❌ Your PR has failed checks. Please review the issues below and take necessary action before merging.

🚦 1 Pipeline job failed

DataDog/datadog-lambda-js | check layer size (node18) — 🔄 Retry may pass, looks flaky

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