Do not give workflow boolean inputs default values

[zcbenz]

There is a pitfall in GitHub Actions that the default value of workflow_dispatch inputs can only be string, so when setting default: true we might get a string "true" or a boolean false as default value (it seems that we ran into the latter case).

This PR just removes the default values so inputs would all default to boolean false for pushes.

Also note that I'm using a non-exist env in name: ${{ inputs.dry_run && 'dry-run' || 'pypi' }} because inputs.dry_run && '' || 'pypi' would always evaluate to pypi regardless of the input value, which is another pitfall.

Checked a few cases and they all seem to work as expected:

• ✅ Manually triggered dry run
• ❌ Manually triggered publish
• ❌ Push triggered publish

[awni]

I'm not sure about removing that: github.event_name == 'push'

I added it because these steps weren't getting triggered because the inputs were not defined when it's a push (vs when it's a manual trigger).

[awni]

If it's not defined will !inputs.dry_run evaluate to true?