feat: support Linux ARM64 by invalidating bundled x64 stamps

[ooaklee]

Merging the upstream-bound feature branch into the fork's default branch so it's available here regardless of when the upstream PR (asdf-community#58 on asdf-community/asdf-flutter) is merged.

See that PR for full context, or the ADR at `docs/adr/adr001-linux-arm64-support.md` for the diagnosis and rationale.

Problem

On Linux `aarch64` hosts, `asdf install flutter ` completes successfully but the first `flutter` invocation fails with:

```
.../bin/internal/shared.sh: line 273:
.../bin/cache/dart-sdk/bin/dart: cannot execute binary file: Exec format error
```

Root cause

Flutter's Linux tarball bundles an x86-64 dart-sdk plus matching `engine-dart-sdk.stamp` and `flutter_tools.stamp`. Because the stamps match on first run, Flutter's self-heal (`update_dart_sdk.sh`, the script that detects host arch and fetches the matching dart-sdk) is never reached. The wrong-arch dart binary stays in place.

`releases_linux.json` exposes no `dart_sdk_arch` field (all 719 entries are `x64`), so the macOS-style archive filtering can't be reused on Linux.

Fix

After extraction, on non-`x86_64` Linux hosts delete both stamps so `update_dart_sdk.sh` runs on first invocation, detects the host arch via `uname -m`, and fetches the matching dart-sdk (e.g. `dart-sdk-linux-arm64.zip`).

The stamp-invalidation logic is extracted into a standalone function (`invalidate_bundled_stamps_for_non_x64_linux`) so it can be unit-tested in isolation. Architecture detection is delegated to Flutter's own `update_dart_sdk.sh`, which already handles `aarch64` and `riscv64` correctly.

Changes

• `bin/install` - new `invalidate_bundled_stamps_for_non_x64_linux()`; top-level execution guarded by a `BASH_SOURCE` check so the file can be sourced by tests; `dirname $0` -> `dirname "${BASH_SOURCE[0]}"`.
• `bin/utils.sh` - drive-by fix: `${VAR:=default}` -> `: "${VAR:=default}"` (the previous form returned exit 127 under `set -e`).
• `test/install.bats` - 7 bats tests covering x86_64 (preserve), aarch64 (remove each stamp), riscv64 (remove both), darwin arm64/x86_64 (preserve), and an unknown-arch default-branch guard.
• `.github/workflows/test.yml` - CI matrix on `ubuntu-24.04` (x86_64) and `ubuntu-24.04-arm` (aarch64).
• `docs/adr/adr001-linux-arm64-support.md` - ADR documenting the context, decision, and consequences.
• `README.md` - new troubleshooting section with a link to the ADR.

Verification

• Reproduced the original `Exec format error` on `aarch64` Linux with `3.44.4-stable`.
• After applying the fix: `file bin/cache/dart-sdk/bin/dart` reports `ELF 64-bit ... ARM aarch64`; `flutter --version` reports `Flutter 3.44.4 • channel stable`; `dart --version` reports `on "linux_arm64"`.
• `bats test/` - 7/7 passing locally on aarch64.