fix(build): Always run tests on main to create reusable caches

[cstepanian]

Pull Request #264 changed the build settings so that tests would not be run on all pushes to the main branch.
Unfortunately, while it seems wasteful to do redundant builds, this has the consequence of preventing re-use of build caches.

The reason is that build caches created for Pull Requests are isolated from other PRs (except those based on that PR) and the main branch. Build caches created for main can be re-used in PR builds, which is what we want.

GitHub Actions docs on re-use of build caches

The good news is that this should save build time overall. It should also reduce churn in the storage of caches: Most of the time, caches made for main can be reused in PR builds, which means that no new caches need to be created at all for PRs that don't change Cargo.toml or Cargo.lock.