Parallelize builds across OS and architecture

Problem

One build-test job builds every OS and architecture for a version. As we add more operating systems, this job needs more disk space and more time.

Workbench hit this limit when we added Ubuntu 26.04. We increased the runner size as a temporary fix.

• https://github.com/posit-dev/images-workbench/pull/190

This problem will return. RHEL 10 support is planned for Workbench and other products. Each new OS adds more work to the same job.

• https://github.com/posit-dev/images-workbench/issues/175
• https://github.com/posit-dev/images-workbench/issues/176
• https://github.com/posit-dev/images-shared/issues/575
• https://github.com/posit-dev/images-shared/issues/698

Scope

This affects two shared workflows: bakery-build-native.yml and bakery-build-pr.yml. All three product repos use these workflows — images-connect, images-package-manager, and images-workbench — including every pull request build.

Goal

Run each OS and architecture combination as its own parallel job, instead of building all of them in one job per version.

See the comments below for the full design. Each comment covers one part.

Start by reading the comments on this issue for the full design, then inspect bakery-build-native.yml and bakery-build-pr.yml. Trace how builds currently cover OS and architecture combinations across the three product repositories; done means each combination runs as its own parallel job for version builds and pull requests.