ci: Build the project on PR pushes

[sprt]

Create a workflow that automatically builds the project on every PR push, so that we can't merge changes that would break deployment.

We should reuse the build job from here:

kata-containers.github.io/.github/workflows/deploy.yml

Line 32 in 054713a

build:

See https://docs.github.com/en/actions/sharing-automations/reusing-workflows