CI: separate alternative pkg builds from main build pipeline

[john-u]

Is your feature request related to a problem? Please describe.
We currently build executables during every release and run functional tests against the major 3 supported platforms (win, macos, linux). However, several other build targets can now be built such as ARMv7, which can be useful for developers, but are only supported by pkg and us on a best-effort basis.

Having such a large list of build targets slows down our main release build. It also adds risk that the release will fail even if only one of the alternative targets has an issue, when the main supported builds could have been tested and released.

Describe the solution you'd like

1. Refactor the release workflow so that we only depend on the successful build of major supported OS pkg targets for functional tests and release.
2. Fork a secondary pkg job that runs best-effort builds and waits for the release to be created before updating it with the alternatives.

Additional context
#409