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

src/main.test.ts

@@ -196,6 +196,36 @@ test("uses versioned tar archives for Supabase CLI v2.99.0 and later", async ()
   });
 });
 
+test("uses apk archives for Supabase CLI v2.99.0 and later on Linux musl", async () => {
+  const { getDownloadArchive } = await getMainModule();
+
+  const archive = await getDownloadArchive("2.100.1", "linux", "x64", true);
+
+  expect(archive).toEqual({
+    url: "https://github.com/supabase/cli/releases/download/v2.100.1/supabase_2.100.1_linux_amd64.apk",
+    format: "apk",
+  });
+});
+
+test("keeps tar archives before Supabase CLI v2.99.0 on Linux musl", async () => {
+  const { getDownloadArchive } = await getMainModule();
+
+  const archive = await getDownloadArchive("2.98.2", "linux", "x64", true);
+
+  expect(archive).toEqual({
+    url: "https://github.com/supabase/cli/releases/download/v2.98.2/supabase_linux_amd64.tar.gz",
+    format: "tar",
+  });
+});
+
+test("uses usr/bin as the CLI path for apk archives", async () => {
+  const { getCliPath } = await getMainModule();
+
+  expect(getCliPath("/tmp/extracted", "apk")).toBe(path.join("/tmp/extracted", "usr", "bin"));
+  expect(getCliPath("/tmp/extracted", "tar")).toBe("/tmp/extracted");
+  expect(getCliPath("/tmp/extracted", "zip")).toBe("/tmp/extracted");
+});
+
 test("keeps the unversioned tar archive layout before Supabase CLI v2.99.0", async () => {
   const { getDownloadArchive } = await getMainModule();