Check package does not exist

[IsabelParedes]

The workflow for building packages should have an additional check that ensures the package hasn't been uploaded already.
The problem is that in some PRs, the build number remains unchanged, but uploading the package does not fail until the PR has already been merged to main. It would be better to detect name conflicts earlier.

Uploading r-lubridate-1.9.4-h072c4ef_0.tar.bz2 for emscripten-wasm32
Error:   × Resource conflict: Failed to upload package file: r-lubridate-1.9.4-
  │ h072c4ef_0.tar.bz2
  │ Status: 409 Conflict
  │ Body: Failed to delete package entry: The package entry already exists