Update continuous-integration-github-actions.md

[fzakaria]

Document how Nix MagicCache is now deprecated.

[fzakaria]

Probably want to remove the whole section but I'll leave that for others to decide; at least more now it's good to recommend not to use it.

[fricklerhandwerk]

I suggest we remove the article and point the URL to a tombstone with the explanation, as we do with release notes about breaking changes in Nixpkgs.

@fzakaria do you happen to have a source to support the claim about what GitHub may or may not want, so we can link to that?

[fzakaria]

@fricklerhandwerk i'm not associated with DetSys; I just linked to their own article how they are deprecating MagicNix and the rationale behind it.

(Unfortunate because it truly was magic and free)

I suggest landing this as is and following up with the removal etc..
For now, no one should be using it as it's been deprecated February 1st onwards.

[fzakaria]

CC @grahamc