Awareness for building a PR (security)

[joonas-fi]

Currently some builds fail if publishing-related ENV var secrets are missing.

We should have awareness that we are building a PR (currently --publish-artefacts is hardcoded in the .travis.yml), and in those cases publishing-related ENV var secrets should be considered optional (because we should not even try to publish artefacts with PRs)

[joonas-fi]

Foundation work started in ab7a0be