The action runs the existing create_release.py script in a new mode
created for github actions. Rather than using local git commands, it
uses a public action for creating a PR in the repository. The action can
be triggered from the website or via the CLI tool, for example
gh workflow run create-release.yml -R emscripten-core/emsdk -F
lto-sha=abc123 -F nonlto-sha=234567
This script is (IMO) more readable, but the real reason for this change is that
it raises an error message when the binary package fails to download. (The shell
script silently generated a bogus hash instead, because the shell's `set -e`
builtin does not affect commands executing inside a $() context.
It seemed just as easy to rewrite the script in Python as to fix that.
This change also updates some outdated filename references.
