Fix environment determination

Author: evertlammerts

.github/workflows/release.yml

@@ -78,7 +78,7 @@ jobs:
           if [[ test == "${{ inputs.pypi-index }}" ]]; then
             ci_env=pypi-test
           elif [[ prod == "${{ inputs.pypi-index }}" ]]; then
-            ci_env=pypi-prod${{ inputs.stable-version && '' || '-nightly' }}
+            ci_env=pypi-prod${{ inputs.stable-version != '' && '' || '-nightly' }}
           else
             echo "::error::Invalid value for inputs.pypi-index: ${{ inputs.pypi-index }}"
             exit 1