Convert Devenv to Nix flake

[srounce]

• Remove Devenv usage, similar functionality with Nix flake allowing easier debugging and Nix REPL usage.
• Slightly loosen Python version requirement
• Ensure dvc cache is local to project, preventing issues on some distros that can't arbitrarily write to /var/...
• Add Direnv config to load new devshell when entering directory
• Enable CI on MacOS runners ensuring environment is tested on both platforms

Supercedes #194 & #195, incorporating the fixes they provide.

[KingMob]

It looks ok afaict, but I'm not a Nix or python expert.

It looks like the Github commit-lint action is broken, though. It says it can't find pnpm. (If it helps, I generally have good luck debugging GH Actions locally with the act tool.)

[srounce]

Yeah I'm having a look into that, it's weird because the step before also uses pnpm and works fine.

[KingMob]

Looks good afaict. I noticed the .envrc is running watch_file $(find . -name '*.nix'), which means it should be looking at .devenv/ and .devenv.flake.nix. I removed them, and it still seems to be working, but should those files be ignored or deleted, to avoid interference?

[KingMob]

LGTM, tho see my note about watching all nix files recursively.

[srounce]

Looks good afaict. I noticed the .envrc is running watch_file $(find . -name '*.nix'), which means it should be looking at .devenv/ and .devenv.flake.nix. I removed them, and it still seems to be working, but should those files be ignored or deleted, to avoid interference?

The Devenv dotfiles can be removed as Devenv is no longer used at all. However well pointed out as I'd forgotten to remove the old unused flake stub in the nix/ directory (now removed).

The watch_file call in the .envrc basically gets Direnv to reload the environment when there has been a change to the nix configuration as it's non-trivial to work out which nix files are related to the devshell. I'm going to add a watch_file to the flake.lock as well as that would also have potentially devshell altering effects.

[KingMob]

@Schaechtle @srounce I just noticed, the docs weren't updated to match. quick-start.adoc and structure-learning.adoc still refer to devenv.

[KingMob]

The Dockerfile also refers to devenv, and many of the Github Action yaml files do, too.