ci: workflow to build example hosts

[montchr]

Would your feature fix an existing issue?

As pointed out in #438 (comment), CI checks can pass but example host configurations can still fail to build. This indicates a gap in our CI workflows.

From that comment:

nix flake check  # this works on the current revision
nix flake show   # this also works on the current revision
nix build .#darwinConfigurations.Mac.system

The last step returns:

error: The option `environment.shellAliases.nrb' has conflicting definition values:
       - In `/nix/store/3qc2jan7bjahmiv9h3p84r1ymkkcla8q-source/profiles/core/common.nix': "sudo > darwin-rebuild"
       - In `/nix/store/3qc2jan7bjahmiv9h3p84r1ymkkcla8q-source/profiles/core/darwin.nix': "sudo > darwin-rebuild switch --flake"

Describe the solution you'd like

I've fixed the direct issue in #438 for the time being, but ideally we should run nix build for the key example hosts in each of the examples.

Since building the host configurations would likely be more time- and resource-intensive than simply running the existing checks, it'd be best to tune the CI workflows to only build hosts when changes to their respective examples could affect the hosts' buildability.

Describe alternatives you've considered

If there's a way to catch errors like the one mentioned above without nix build, that would probably be preferable.