Check if packages have been really built before trying to compute their hashes - Fixes #4286

[crisbal]

This should fix #4286

[anaconda-issue-bot]

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[jezdez]

@anaconda-issue-bot check

[crisbal]

@anaconda-issue-bot check

[github-actions[bot]]

Hi there, thank you for your contribution!

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed automatically if no further activity occurs.

If you would like this pull request to remain open please:

1. Rebase and verify the changes still work
2. Leave a comment with the current status

NOTE: If this pull request was closed prematurely, please leave a comment.

Thanks!