Remove unnecessary dependencies from the "Update builder" job

[edmorley]

Currently the "Update the builder" job runs at the very end, since it's declared as having a dependency on all of the other jobs:

flowchart LR
    A[Compile buildpacks] --> B[Publish to Docker Hub]
    B --> C[Publish to CNB registry]
    A --> D[Create GitHub release]
    C & D --> E[Update builder]

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However, opening the builder PR only actually needs the "Publish to Docker Hub" step to have been completed. ie:

flowchart LR
    A[Compile buildpacks] --> B[Publish to Docker Hub]
    B --> C[Publish to CNB registry]
    A --> D[Create GitHub release]
    B --> E[Update builder]

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By removing these unnecessary dependencies from the workflow definition, we:

1. Ensure that we can still publish builder updates if the CNB registry is having downtime (xref https://salesforce.quip.com/QRwnAaGborAF#temp:C:RCc6106ab8f1a3e4f37968a5b44a)
2. Speed up the end to end time of publishing a CNB, since more of the jobs can now run in parallel

However, we might want to wait to do this until we've done #113, since currently the only signal buildpack maintainers have for failed jobs is that the builder update PR never gets opened - so we want to make sure there is still a way to see that eg the CNB registry job has failed.

GUS-W-13956414.