* Seems like whatever issue was preventing us from evaluating
derivations for other architectures (without actually building them)
has been resolved, and now we can nicely test everything with a single
invocation of `nix flake check`
* `nix flake check` insists on evaluating all outputs for all systems,
which seems to include evaluating build inputs needed for other
systems (which gives us errors like `a 'aarch64-darwin' with features
{} is required to build '...', but I am a 'x86_64-linux'`)
* To work around this we'll move all tests to their own directory
outside of the flake's outputs, and manually invoke a test script to
build them
