ci(refactor): split build-and-test-pkg for better reuse

Issue #, if available:

Description of changes: While I was fixing the macOS pkg build error on macOS 11 (due to yq from brew not building properly), I noticed that most of the file was repeated (due to different archs being built/tested as different jobs) which caused it to be pretty unnecessarily unwieldy. This was the case for both the build and the test workflow jobs.

I initially thought I could combine them using matrices, but then we lose the ability to run the test jobs as soon as each respective build completes (with a matrix, it would have to wait until both builds complete, which isn't even guaranteed). There were other complications that made matrices hard, like the fact that we only want to build once per arch, but we want to test on several other arch/version combinations.

Instead, I was able to split the workflows into reusable/composite workflows, so that only the inputs vary, and the script is mostly the same.

Testing done:

[x] I've reviewed the guidance in CONTRIBUTING.md

License Acceptance

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

runfinch / finch

ci(refactor): split build-and-test-pkg for better reuse #953

License Acceptance