CI: speed up GitHub actions

[mmilata]

Aside from the tests themselves, there are some things that will speed up everything:

1. cache nix environments
   • https://github.com/cachix/cachix-action
   • https://github.com/nix-community/cache-nix-action
   • https://github.com/DeterminateSystems/magic-nix-cache-action
2. cache poetry environments (or implement #2271 and 1.)
3. investigate why uploads to data.trezor.io are so slow
   • every filename is logged and there are lots, there's probably some option in awscli not to print them