feat(installer): add portable setup option

[tkkhq]

Tracking

• Related: https://github.com/Kong/volcano-web/pull/526
• Related: https://github.com/Kong/volcano-web/pull/520

Why

• clients that install the CLI and immediately run volcano setup cannot rely on the installed directory being present in the current shell's PATH
• the release installer requires Bash even though a POSIX shell can support its behavior
• clients must duplicate the installer's path selection to run setup through the installed binary

Summary

• make the release installer compatible with POSIX sh
• add an installer --setup option that runs setup through the resolved installed binary path
• warn before setup when volcano in PATH points to another binary
• reject unknown installer options before downloading a release
• test shell and package-manager selection for every OS and architecture published in GitHub Releases
• document the portable install and combined setup commands

Verification

• sh -n scripts/install-volcano.sh scripts/install-volcano.test.sh
• sh scripts/install-volcano.test.sh
• shellcheck -s sh scripts/install-volcano.sh scripts/install-volcano.test.sh
• node --test scripts/npm/download.test.js
• make check
• confirmed the latest GitHub release publishes Linux AMD64/ARM64, macOS AMD64/ARM64, and Windows AMD64 binaries

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