fix: Upgrade standalone turbo on installer reruns (#14330)
## Summary
- Replace a regular binary at the standalone installer destination after
verifying the release archive, without changing binaries found elsewhere
on PATH.
- Report `Upgraded turbo <version> at <path>` on reruns and `Installed
turbo <version> at <path>` on fresh installs.
- Preserve the existing binary if verification fails, and reject
symlinks at the destination.
- Show the existing PATH location and a platform-specific rerun command
when another installation blocks the standalone installer, without
guessing how to remove a package-manager-owned binary.
```text
rerun installer
binary at selected destination → verify archive → replace → "Upgraded turbo ..."
binary elsewhere on PATH → show path + uninstall guidance + rerun command
symlink at destination → refuse to overwrite
```
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Co-authored-by: vercel[bot] <35613825+vercel[bot]@users.noreply.github.com>
Co-authored-by: Vercel <vercel[bot]@users.noreply.github.com>
