Update CI to build for all matrix targets

[eager-signal]

We noticed that the CI matrix wasn’t always building for the Rust target it intended, leading to surprise breakage (e.g. #187) when doing release builds. This addresses that in a few ways:

1. add --target to cargo test
2. check_only flag
   • skip running tests for non-native architectures and targets; instead do cargo build … --tests to confirm cross-compilation
3. various customization options
   • specific packages required to cross-compile for the target, environment variables, etc.
4. last resort: drop a target
   • musl doesn’t have a standard C++ setup to use

[nox]

Thanks for the impressive PR! Can you please rebase on top of master now that #187 is merged? Thanks.

[eager-signal]

Can you please rebase on top of master now that #187 is merged? Thanks.

@nox yep, done!

[jrose-signal]

Fixes #62 :-)