Ensure script/release.sh is idempotent

[joshmgross]

The assumption in #1234 that GoReleaser was idempotent was not correct, the workflow failed when attempting to republish existing assets to the GitHub Release -

  ⨯ release failed after 6m53s                       error=scm releases: failed to publish artifacts: failed to upload pscale_0.278.0_macOS_amd64.tar.gz: POST https://uploads.github.com/repos/planetscale/cli/releases/303388482/assets?name=pscale_0.278.0_macOS_amd64.tar.gz: 422 Validation Failed [{Resource:ReleaseAsset Field:name Code:already_exists Message:}]

https://github.com/planetscale/cli/actions/runs/23766950223/job/69248755043

GoReleaser does have a replace_existing_artifacts option, but I do not want to replace existing published releases. We can't avoid replace_existing_artifacts as far as I can tell - 844cf16

This adds some level of idempotency to script/release.sh by checking if a version already exists for each publish target.

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