Verification github action for PR

[myShoggoth]

With the new changes in the hakyll branch, we have an infrastructure that can build and then deploy to the main branch.

Next we need the same kind of CI for PRs into that branch that does the build, and hopefully later some verification steps, but no deploy.

[visortelle]

I enabled build for all branches instead of only for hakyll. Also I added conditional deployment step skiping for pull request in this PR:

https://github.com/haskellfoundation/haskellfoundation.github.io/pull/170

How it looks on CI:

Example here:

https://github.com/visortelle/haskellfoundation.github.io/runs/4666439616?check_suite_focus=true