Add --local-engine-host, which if specified, is used instead of being inferred

[matanlurey]

Closes #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

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

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

After

$ 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

[jonahwilliams]

LGTM!

[gaaclarke]

Didn't do a code review, but wanted to say this approach sounds good to me. It's mutually exclusive to your ideas about changing the default behavior and having better error messaging. We could chose to do those too if we wanted in the future.

[matanlurey]

Didn't do a code review, but wanted to say this approach sounds good to me. It's mutually exclusive to your ideas about changing the default behavior and having better error messaging. We could chose to do those too if we wanted in the future.

Thanks! I appreciate you writing this!

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