This removes one dependency of the project, by moving the Nix cache
store into Github. There is no expected speed up, except for lower
latency between the actions runner and the cache store.
This relies on Cargo2Nix, and because we don't want to be maintaining
that file by hand, we introduce a new continuous workflow to
automatically update the generated Cargo.nix and submit a PR whenever
something is merged into master.
This should make the process of supporting Nix bearable. We'll
re-assess in a while.
Co-authored-by: KtorZ <matthias.benkort@gmail.com>
KtorZ