Make sure Contributing building GitHub Actions is up to date

[jherzstein]

1. The github actions workflow is called "build" not "build-secureblue"
2. yml file is in .github/workflows/build-all.yml
3. Add kernel signing key to docs.
   a. For testing the privkey in https://github.com/secureblue/secureblue/blog/live/.github/workflows/private_key.priv.test can be used for KERNEL_PRIVKEY repository sercret.
   b. Move test pubkey https://github.com/secureblue/secureblue/blog/live/.github/workflows/pub_key.der.test to files/system/etc/pki/akmods/certs
4. Make sure that you are set as github.triggering_actor.
5. Change .github/workflows/shared-runner-setup/action.yml references to ghcr.io/secureblue to ghcr.io/YOUR_GITHUB_USER No longer looks needed, see ci: improve fork development secureblue#2037.
6. For updates, you need to have your cosign public signing key in /etc/pki/containers/ called YOUR_GITHUB_USER-2025.pub.