Add dfinity/setup-dfx to GitHub Actions Marketplace

[viktorkovarik]

When you've recently released breaking change in dfx (dfxvm) which broke aviate-labs/setup-dfx I've realized, that your GH action is not available on GitHub Actions Marketplace only the aviate-labs.

FYI, I've risen request to deprecate aviate-labs/setup-dfx aviate-labs/setup-dfx#8 (comment) so I hope that it won't confuse users anymore.

[frederikrothenberger]

I second this. Having proper releases with actual versions would be nice instead of having to pin specific commits.