I think Travis does build also on pushes to master
That wasn't really what I was meaning, what I meant was that when a PR is merged the other PRs are not re-tested to be sure they still compile/work with new master, eg if PR A removes variable X declaration, but PR B uses X, then merging A will make B fail to compile, but I don't think Travis notices on PR B if it already exists when A is merged.
For Linux, yeah I said its hopeful :) But flatpacks or snaps may not be good for testing since they are tied to the dependencies in the package. For testing we want a wide range of environments to be used, maybe forget it.
For windows, I didn't realise that the installer was built on actual windows, not the cross build, so I guess it depends on how much work it is to get going on the cross build to decide if its worth it.