Reduce flakiness of CI tests

[mattyg]

Reduce flakiness of ci tests by:

• Using cachix key with write permission to reduce nix environment setup time keep flakes.lock up to date
• Refactor tests to avoid using sleep() calls (waiting on Feature Request: function that waits until all agents have identical dht state holochain/tryorama#167)

[jost-s]

Tests have been running stably on recent branches after increasing the zome call timeout.