update nix flake environment (#654)

# Objective

I noticed that when trying to diagnose an issue that we don't compile
with the current nix lock, (rust being to old)
```
error: package `tokio-macros v2.4.0` cannot be built because it requires rustc 1.70 or newer, while the currently active rustc version is 1.65.0-nightly
```

# Solution
ran `nix flake update`, and now it brings in a recent enough rust
nightly for everything to compile