Add support for linux/arm64

Recognize the linux/arm64 platform so the CodeQL CLI is accepted on
Arm64 Linux runners and the native per-platform bundle
codeql-bundle-linux-arm64.tar.zst is resolved, rather than falling
through to the combined all-platforms gzip fallback (which deliberately
omits arm64).

Add ["linux", "arm64"] to the supported platforms and make the linux
bundle name arch-aware. Keep the download mock faithful and cover the
new arch branch with tests.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

src/cli-errors.test.ts

@@ -128,7 +128,6 @@ test("CliError constructor with empty stderr", (t) => {
 
 for (const [platform, arch] of [
   ["weird_plat", "x64"],
-  ["linux", "arm64"],
   ["win32", "arm64"],
 ]) {
   test.serial(