Add regular CI tests to verify full demo binaries build for all targets

[adamgreig]

Occasionally nightly Rust can have breakages in certain targets; we could catch these automatically by extending our existing weekly CI job to include building some full example binaries and possibly running in qemu. See rust-embedded/wg#941 (comment), rust-embedded/aarch32#227, https://github.com/ferrous-systems/rust-training/blob/main/.github/workflows/weekly-canary-build.yml.

[thejpster]

And https://github.com/rust-embedded/embedded-alloc/blob/master/.github/workflows/ci.yml