Update CI workflow to remove the if that disables running on PRs

[nils-a]

In cake-contrib/Cake.Recipe#809 we established that the if in

# ...
build:
    runs-on: ${{ matrix.os }}
    if: github.event_name == 'push' || github.event.pull_request.head.repo.full_name != github.repository
# ...

does more harm than good, and should be replaced by limiting the branches which are build on push:

push:
  branches:
    - main
    - develop
    - "feature/**"
    - "release/**"
    - "hotfix/**"
    - "support/**"
  tags:
    - "*"