ci: drop redundant pnpm bootstrap on arm64 publish (#9345)

## Summary

- The arm64 build leg of the publish workflow runs `curl -fsSL
https://get.pnpm.io/install.sh | sh -` to bootstrap pnpm. That installer
always pulls the **latest** pnpm; **11.0.8** dropped today and `pnpm
setup` now hard-errors when `PNPM_HOME` is not in `PATH`, breaking the
`v1.43.0-rc.0` publish: [run 25559324365 / job
75029848553](https://github.com/ChainSafe/lodestar/actions/runs/25559324365/job/75029848553).
- v1.42.0-rc.0 worked because the installer pulled pnpm 10.33.0
([previous
run](https://github.com/ChainSafe/lodestar/actions/runs/23947458219)).
- The very next step (`./.github/actions/setup-and-build`) already runs
`pnpm/action-setup@v4`, which honors the `packageManager` field in
`package.json` (currently pinned to `[email protected]`) and installs pnpm
correctly — making the curl bootstrap redundant.

## Test plan

- [ ] Re-tag / re-trigger the publish workflow (or run `Build binaries`
via `workflow_dispatch`) and confirm the arm64 leg completes through the
pnpm setup, install, and caxa packaging steps.
- [ ] Verify the produced arm64 tarball still passes the `Sanity check
binary` step.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: lodekeeper <[email protected]>

Author: lodekeeper

.github/workflows/binaries.yml

@@ -31,8 +31,6 @@ jobs:
       - name: Install arm64 specifics
         if: matrix.arch == 'arm64'
         run: |-
-          # Install missing pnpm
-          curl -fsSL https://get.pnpm.io/install.sh | sh -
           # Install missing build-essential
           sudo apt-get update
           sudo apt-get install -y build-essential python3