fix multi-wheel output

[casperdcl]

when there are multiple dist/*.whl (e.g. when using cibuildwheel or cross-compilation), this fails:

echo "whl=$(ls *.whl || :)" >> $GITHUB_OUTPUT

with error:

Error: Unable to process file command 'output' successfully.
Error: Invalid format '<second_wheel_name>.whl'

Solution: https://docs.github.com/en/actions/writing-workflows/choosing-what-your-workflow-does/workflow-commands-for-github-actions#multiline-strings