factorio: Fix bug in update.py
factorio: Fix bug in update.py

Prior to this change, `update.py` fails with `KeyError: 'alpha'` when upstream
does not have a version in the `experimental` channel:

	./update.py --username XXXXXXXX --token "$(wl-paste)"
	Traceback (most recent call last):
	  File "./update.py", line 180, in <module>
	    app.run(main)
	  File "/nix/store/854z51wg8fj1s6ghxasp1xxx8khb3r8v-python3-3.8.9-env/lib/python3.8/site-packages/absl/app.py", line 303, in run
	    _run_main(main, args)
	  File "/nix/store/854z51wg8fj1s6ghxasp1xxx8khb3r8v-python3-3.8.9-env/lib/python3.8/site-packages/absl/app.py", line 251, in _run_main
	    sys.exit(main(argv))
	  File "./update.py", line 162, in main
	    new_our_versions = generate_our_versions(factorio_versions)
	  File "./update.py", line 92, in generate_our_versions
	    version = factorio_versions[release_channel.name][release_type.name]
	KeyError: 'alpha'

I chose to make it so that the versions in nixpkgs' `factorio-*` packages are
either experimental (when available) or stable, as this seems like the most
coherent approach

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