build: remove dependency on asset/ directory existence.

[chaosteil]

This PR simplifies the handling with the asset/ directory. Just a few less instructions for someone to follow who wants to quickly publish a release.

Note than in windows instead of silently passing through, it force-creates an empty asset/ directory instead (and does not error if it does already exist), but which still results in an empty zip file with only the binary.

Tested that it works by running the template with the assets present: https://github.com/chaosteil/bevy_github_ci_template/actions/runs/8217244209 resulting in release https://github.com/chaosteil/bevy_github_ci_template/releases/tag/v0.0.1-test

and as well as running the template without the assets directory present:
https://github.com/chaosteil/bevy_github_ci_template/actions/runs/8217285995 resulting in release https://github.com/chaosteil/bevy_github_ci_template/releases/tag/v0.0.1-test-assetremove

[BD103]

Looks good, though I did not test it locally.

I would recommend adding comments on why it's || true all of the cp commands, but I won't block on it. :)