Add a build.build-target config key

[phil-opp]

This key allows overriding the default target for build operations (e.g. cargo build, cargo check, cargo run), but not for test operations (cargo test, cargo bench).

See #6784 and https://internals.rust-lang.org/t/set-default-target-for-cargo-build-but-not-for-cargo-test/9777 for motivation.

Fixes #6784

[rust-highfive]

r? @ehuss

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[alexcrichton]

Thanks for the PR! This seems like a sort of odd feature to me though... The naming "build target" makes me think that $HOST is being changed for build scripts, but then I was surprised to learn that it's quite different from what this PR is implementing. I'm not really sure if there's a good name though for this, and I'm not really sure how to rationalize the feature myself...

I suspect though this is something we'd want to for sure land as unstable rather than insta-stable

[phil-opp]

Thanks for taking a look! I agree that setting different default targets for cargo build and cargo test is probably a niche use case. I'm of course happy to change the name and implement it behind a feature gate.

I'm currently exploring an alternative solution to my problem: Custom test frameworks (rust-lang/rust#53410) might allow to run cargo test for the bare metal target system too. If this works, I probably don't need the feature of this PR anymore. I will report back soon!

[phil-opp]

I managed to get custom test frameworks working, so I'm now able to run cargo test for the target system too. I no longer need the feature of this PR, so I'm closing it.