Closes https://github.com/flutter/flutter/issues/132162.

I did a tiny bit of minor cleanup but didn't want to go 🍔 beyond the
scope of this change. After it lands I'll update the Wiki.

## Before

```bash
$ fl run \
  --local-engine-src-path=$ENGINE \
  --local-engine=android_debug_unopt_arm64
```

... would try to use `host_debug_unopt` (i.e. Rosetta).

## After

```bash
$ fl run \
  --local-engine-src-path=$ENGINE \
  --local-engine=android_debug_unopt_arm64 \
  --local-engine-host=host_debug_unopt_arm64
```

... uses `host_debug_unopt_arm64`, as specified.

---

/cc @jonahwilliams @gaaclarke @zanderso