Enable CIs on pull requests

[hmaarrfk]

closes #100

[mverleg]

I think I've disabled those as a default because it's not always safe to run Actions on not-yet-approved code, since they have access to secrets etc. But this repository actually doesn't seem to have such secrets, so it's probably fine. Thanks for the change!

[hmaarrfk]

I think that secrets are typically only enabled for PRs that come from "protected" branches and such.

COntributors would fork, and typically won't have access to the secrets.

[hmaarrfk]

this was my understanding on how many of these CI pipelines were structures from different providers.