Clean-up GitHub actions

I think we don't need to clean `/opt` anymore, the VM size has been increased.

I also think we don't need to run checks on the currently used PR events: they're already being run on push.

It could instead be useful to run a build when the PR is merged - but again, it seems like that would happen anyway with the `push` selector. I'm just not sure if it would necessitate a merge commit.

Ideally, we would run checks on the merged PR **before** it is merged (Travis does this) - can we do that here?

Inspect the repository's GitHub Actions workflow configuration and compare the current push and pull-request event selectors. Determine whether merged-PR builds and pre-merge checks are covered without the /opt cleanup, then confirm the workflow triggers match the intended behavior.

Written by the indexing model from the issue text.