Merge separate CI configurations to reduce copy/pasta 🍝 of etcd versions

https://github.com/etcd-io/jetcd/pull/1481 made me have a closer look

at https://github.com/etcd-io/jetcd/blob/main/.github/workflows/build-main.yml

vs https://github.com/etcd-io/jetcd/blob/main/.github/workflows/build-pr.yml

and it seems to me that we could actually merge them... @lburgazzoli or am missing anything?

I could raise a PR, but probably easier (less rebate/merge overhead, for me) if I do it AFTER #1481 is merged (or closed).

PS: https://github.com/etcd-io/jetcd/issues/1486 for background.

Compare .github/workflows/build-main.yml and .github/workflows/build-pr.yml, using pull request #1481 and issue #1486 for context. Check whether their differences can be consolidated without losing the intended CI coverage or etcd-version handling. Done means one maintainable configuration replaces the duplicated setup and the relevant workflows still pass.

Written by the indexing model from the issue text.