CI: Nix caching

[t-wallet]

Currently, the nix-build job does not implement any caching. It just fetches the packages and builds them from scratch every time. Caching dependencies would greatly reduce the time of this CI job. There are a couple of options:

• Explore implementing caching locally. We have tried this, but we haven't got it to work.
• Use cachix. If we need more than 5 GB of storage, we need to pay.