fix: setup-cli on Linux musl containers (#431)

## Summary

Fixes setup-cli in Alpine/Linux musl containers after the Supabase CLI
v2.99+ release layout introduced a Bun/TypeScript `supabase` shim
alongside `supabase-go`.

This keeps the v2 action on Bun, but makes the musl paths explicit:

- Detect Linux musl before running `oven-sh/setup-bun` and pass the
matching `bun-linux-*-musl.zip` via `bun-download-url`.
- Use POSIX `sh` for composite shell steps so the action can run in
minimal Alpine containers without `bash`.
- Detect Linux musl in the CLI installer and download the existing
`.apk` release asset for CLI v2.99+.
- Add the extracted APK `usr/bin` directory to PATH.
- Keep existing tar/zip behavior for glibc Linux, macOS, and Windows.

## Root Cause

`oven-sh/setup-bun` does not currently include libc detection in its
automatic release asset selection, so Alpine containers received a glibc
Bun binary. Separately, setup-cli downloaded the generic Linux `.tar.gz`
Supabase CLI asset, whose `supabase` shim is glibc-linked in v2.99+; the
`.apk` asset contains the musl-linked shim.

## Testing

- `bun run ci`
- Manual GitHub workflow in
https://github.com/jgoux/setup-cli-testing/actions/runs/2616196598653