fix(NODE-6642): include source build script in published package

[tadjik1]

Description

Summary of Changes

etc/install-zstd.sh was missing from the files array in package.json, so it was never included in the published npm package.

When prebuild-install cannot find a prebuilt binary (e.g. Node 24 on Windows, or any unsupported platform/architecture combination), the install falls back to npm run clean-install → npm run install-zstd → bash etc/install-zstd.sh. Without the script present, this fails with:

bash: etc/install-zstd.sh: No such file or directory
exit code 127

This fix adds etc/install-zstd.sh to the files array. etc/docker.sh is intentionally excluded as it is a CI utility not needed for package installation.

Double check the following

• Lint is passing (npm run check:lint)
• Self-review completed using the steps outlined here
• PR title follows the correct format: type(NODE-xxxx)[!]: description
  • Example: `feat(NODE-1234)!: rewriting everything in coffeescript`
• Changes are covered by tests
• New TODOs have a related JIRA ticket

[Copilot]

Pull request overview

This PR fixes the npm package contents by ensuring the source-build fallback script (etc/install-zstd.sh) is included in the published tarball, so prebuild-install fallback installs don’t fail when no prebuilt binary is available.

Changes:

• Add etc/install-zstd.sh to the files allowlist in package.json so it is published to npm.

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[tadjik1]

Duplicate of #110 which has the identical fix. Closing in favour of that PR.