Enable branch builds on free GitHub Actions runners

[aaylward]

Replaces 'ubuntu-latest-4-cores' with 'ubuntu-latest' in branch.yml and adds Bazel resource limits (4GB RAM, 2 CPUs) to prevent OOM errors on standard runners. This removes the dependency on paid runners for branch builds.

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PR created automatically by Jules for task 8215646250712213223 started by @aaylward

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