ci: remove Magic Nix Cache

[danth]

Due to the large number of derivations we are building, this almost always gets disabled due to rate limits before it's able to fetch anything which isn't already on cache.nixos.org. Rate limiting also spams the log with warnings which are unrelated to the build.

[trueNAHO]

Getting lots of rate-limiting failures described in the usage notes. could this be addressed by using an optional alternate github token with a higher rate limit like the one used by nix-installer-action?

-- DeterminateSystems/magic-nix-cache-action#28

[trueNAHO]

Due to the large number of derivations we are building, this almost always gets disabled due to rate limits before it's able to fetch anything which isn't already on cache.nixos.org. Rate limiting also spams the log with warnings which are unrelated to the build.

The main reason we recently hit the rate limit all the time is due to the enormous amount of backports done, which is not very representative of "normal" usage.

FYI, I am currently working on a patch to significantly reduce our CI time by only testing the derivations that are relevant to the PR. This might further reduce rate limit hits.