Examples not built in CI

[rylev]

It's currently possible to break examples without CI failing. We should add a test in CI that builds all examples. Because examples are run using various tools that are not installed by default on the CI runner, we might want to take advantage of the e2e test docker image to build the examples.

[itowlson]

Funnily enough build.rs does build the specific case of the timer trigger example, but not the application examples. (I think the trigger example was identified as being sensitive to some of the plumbing work.) The script does examine an environment variables called BUILD_SPIN_EXAMPLES but it looks like that mostly controls building some of the integration tests...?

[endocrimes]

We probably want to purge a lot of that from build.rs and improve the tooling for running them independently - it would definitely make a lot of packaging and installation cases simpler