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
52a6de2558