Fix: Nix flake fails to run cargo +nightly fmt

[Zantier]

Currently

If you take the following steps:

• Install nix.
• Create /.envrc file with contents "use flake".
• Run direnv allow (the rust toolchain is then downloaded).

Running pre-commit run --all-files or cargo +nightly fmt (given in .pre-commit-config.yaml) fails with:

error: no such command: `+nightly`

	Cargo does not handle `+toolchain` directives.
	Did you mean to invoke `cargo` through `rustup` instead?

And I can see that the nightly toolchain is needed, because without it, cargo fmt gives:

Warning: can't set `imports_granularity = Module`, unstable features are only available in nightly channel.
Warning: can't set `group_imports = StdExternalCrate`, unstable features are only available in nightly channel.
Warning: can't set `reorder_impl_items = true`, unstable features are only available in nightly channel.
Warning: can't set `imports_granularity = Module`, unstable features are only available in nightly channel.
Warning: can't set `group_imports = StdExternalCrate`, unstable features are only available in nightly channel.
Warning: can't set `reorder_impl_items = true`, unstable features are only available in nightly channel.

This PR

This update will not affect anybody not using nix.

This PR changes the nix flake as follows:

• Simply use rustup instead of installing a specific version of cargo.
• Remove shell.nix:
  • Make the flake simpler.
  • The flake will refresh when you update flake.nix, but seems to incorrectly use the outdated cached shell when you update shell.nix.
• Activate the python venv automatically when you cd to this repo. (The venv still needs to be created manually: see README.md).

PR Tests

I have tested that the following commands run successfully:

• make build-extension-debug
• pytest
• cargo test
• make test
• pre-commit run --all-files
• make pcc