Attempt to add multi-platform builds (copilot-agogo)

[paullaycock]

Developing on a mac, it was painful not to have the arm images and have a working but not working status through emulation.. it would be great to at least have the py311 base image have a cross-platform build, this is intended to be a generic fix

[Soap2G]

Thanks @paullaycock, cross-platform builds would be indeed useful.

A few thoughts:

1. This changes the shared build-and-push-image step used by the whole matrix, so it'll apply to whatever directory happens to be touched on a future push

There are a few environments that would break under linux/arm64:

• vre-singleuser-root — base is jupyter/scipy-notebook:python-3.9.5, which has no arm64 manifest at all (confirmed via the Docker Hub registry API). Build fails right at FROM.

• atlas-ntuples — same problem, rootproject/root:6.24.06-ubuntu20.04 is amd64-only.

• vre-singleuser-dask-root — FROMs vre-singleuser-root, inherits both issues.

2. We could scope platforms: per matrix entry; I've added a suggestion that only adds arm for specific images.
   (I may have messed up the indentation, but the idea is there)

3. Last thing: env-build.yaml triggers only on push to main, i.e. there's no way to test this before merging the PR. I've suggested to add a test build on PRs.

[paullaycock]

Going with opt-in as discussed in person.

Also fixed a bug: the pull_request: trigger was indented as a stray top-level key instead of nested under on:, so PR-triggered test builds weren't actually running (GitHub Actions silently ignores unrecognized top-level keys). Nested it under on: alongside push: - PR builds should now actually trigger.