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

[vorburger]

#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: #1486 for background.

[lburgazzoli]

What do you mean by mergin them ?