Add architectures to desktop integration test matrix, and enable arm64/x86 tests. (#931)

Add *arch* to the build_desktop matrix, and add architecture tags to all of the artifact names etc.

This allows us to:
- Actually use the architecture_\* fields in print_matrix_configuration.py
- Enable arm64 Mac tests on our M1 Mac GitHub custom runner (when testing on the expanded matrix).
- Enable x86 Linux/Windows tests (when testing on the expanded matrix).
  - For Linux, this required moving the x86 runner setup to a shared module and add customization options to the install prereqs script.

This PR includes all of the above. And look, it reveals some bugs! e.g. an x86 issue in RTDB (fixed in #939)

Author: jonsimantov