Move and revamp workflows, and add publishing workflow

[agriyakhetarpal]

Description

This PR makes some final arrangements before the 0.9 release (https://github.com/pyodide/micropip/milestone/2).

• Pinned all GitHub Actions to their commit hashes
• Switches to a GitHub release instead of a tag for publishing to PyPI (more stringent as it's harder to create a release than to push a tag)
• Moved PyPI deployment job to a separate workflow that builds the sdist + wheel in a job and uploads them in a separate job