Release workflow incorrectly overwrites existing release binaries

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We have occasionally dealt with botched releases where release-please is confused about which version it is releasing. It attempts to re-release a version that has been released. For the most part this is side-effect free, but there is at least one exception: release artifacts on the Github release may be changed. And, in the case of our Swift package, this can cause the SHA in Package.swift to no longer match the one on the Github release.

We should remedy this by making sure that we don't re-upload artifacts if a release is already created prior to the release workflow.