ci: inline Alpine node setup to fix flaky linux-musl CI

[jamescrosswell]

Summary

The linux-musl-arm64 "Initialize Alpine Linux" step has been failing on almost every push for the last few days (#5408). It piped a network-fetched script straight into a shell:

curl -sSL https://raw.githubusercontent.com/.../.github/alpine/setup-node.sh | sudo bash /dev/stdin

With no --fail/retry and a raw pipe, a rate-limited or truncated fetch got executed as-is, failing with line 2: syntax error near unexpected token 'newline'. It's non-deterministic — the same script passes and fails on adjacent commits.

Detailed explanation

GitHub's Actions runner has a hard limitation: JavaScript actions (like actions/checkout) inside Alpine containers are only supported on x64 Linux runners. On the linux-musl-arm64 job (Alpine on ARM), the runner normally refuses to run any JS action.

Our build.yml contained a workaround that runs before Checkout, because checkout is a JS action — it needs the workaround in place first.

run: |
  curl -sSL https://raw.githubusercontent.com/${{ github.repository }}/${{ github.sha }}/.github/alpine/setup-node.sh | sudo bash /dev/stdin

This downloaded a script (setup-node.sh) from GitHub's raw CDN and piped it straight into sudo bash. That script contained:

if [ "$(uname -m)" != "x86_64" ]; then
    for node in /__e/node*; do
        mkdir -p $node/bin
        ln -s /usr/bin/node $node/bin/node
        ln -s /usr/bin/npm  $node/bin/npm
    done
    sed -i 's/ID=alpine/ID=unknown/' /usr/lib/os-release
fi

The downloading of the script is the bit that frequently fails - the script itself runs fine.

Solution

The solution is to move the logic that the downloaded script would have run directly into our build.yml file... to avoid the flaky download. So the build workflow does exactly the same thing as before, but without a flaky network dependency.

Issues

Fixes #5408

#skip-changelog

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.70%. Comparing base (2f0203f) to head (d8fe83a).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5455      +/-   ##
==========================================
+ Coverage   74.68%   74.70%   +0.01%     
==========================================
  Files         512      512              
  Lines       18722    18722              
  Branches     3660     3660              
==========================================
+ Hits        13983    13986       +3     
+ Misses       3866     3864       -2     
+ Partials      873      872       -1     

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