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

action.yml

@@ -15,19 +15,55 @@ outputs:
 runs:
   using: composite
   steps:
+    - id: bun-download
+      name: Resolve Bun Download URL
+      shell: sh
+      working-directory: ${{ github.action_path }}
+      run: |
+        set -eu
+
+        if [ "${RUNNER_OS}" != "Linux" ]; then
+          exit 0
+        fi
+
+        # setup-bun does not detect Linux musl yet, so Alpine-like containers need the musl asset explicitly.
+        is_musl=false
+        if [ -f /etc/alpine-release ]; then
+          is_musl=true
+        elif command -v ldd >/dev/null 2>&1 && ldd --version 2>&1 | grep -qi musl; then
+          is_musl=true
+        fi
+
+        if [ "${is_musl}" != "true" ]; then
+          exit 0
+        fi
+
+        version="$(cat .bun-version)"
+        case "$(uname -m)" in
+          x86_64) arch="x64" ;;
+          aarch64|arm64) arch="aarch64" ;;
+          *)
+            echo "Unsupported Linux musl architecture: $(uname -m)" >&2
+            exit 1
+            ;;
+        esac
+
+        echo "url=https://github.com/oven-sh/bun/releases/download/bun-v${version}/bun-linux-${arch}-musl.zip" >> "$GITHUB_OUTPUT"
+
     - name: Setup Bun
       uses: oven-sh/setup-bun@0c5077e51419868618aeaa5fe8019c62421857d6 # v2.2.0
       with:
         bun-version-file: ${{ github.action_path }}/.bun-version
+        bun-download-url: ${{ steps.bun-download.outputs.url }}
 
     - name: Install Action Dependencies
-      shell: bash
+      shell: sh
       working-directory: ${{ github.action_path }}
       run: bun install --frozen-lockfile --production
 
     - id: setup-cli
       name: Setup Supabase CLI
-      shell: bash
+      shell: sh
       working-directory: ${{ github.action_path }}
       env:
         INPUT_VERSION: ${{ inputs.version }}