Update dependencies (#858)

* flake.lock: Update, exclude Nixpkgs

Updating Nixpkgs seems to break the Nix build somehow (even though we
don't use the Nix input except for its `binaryTarball` job...).

Flake lock file updates:

• Updated input 'fenix':
    'https://api.flakehub.com/f/pinned/nix-community/fenix/0.1.1727%2Brev-66fc1883c34c42df188b83272445aedb26bb64b5/018cc416-f0ae-724d-989f-aa4ded05d885/source.tar.gz?narHash=sha256-Vti1mv4WhmXHPNcFgUiJyt4OKLvsvLzM2eKS4bEegf0%3D' (2024-01-01)
  → 'https://api.flakehub.com/f/pinned/nix-community/fenix/0.1.1762%2Brev-668102037129923cd0fc239d864fce71eabdc6a3/018d63bb-6455-7a2f-98c6-74a36b8216a4/source.tar.gz?narHash=sha256-4o6TMpzBHO659EiJTzd/EGQGUDdbgwKwhqf3u6b23U8%3D' (2024-02-01)
• Updated input 'fenix/rust-analyzer-src':
    'github:rust-lang/rust-analyzer/cf52c4b2b3367ae7355ef23393e2eae1d37de723' (2023-12-31)
  → 'github:rust-lang/rust-analyzer/42cb1a2bd79af321b0cc503d2960b73f34e2f92b' (2024-01-31)
• Updated input 'nix/libgit2':
    'github:libgit2/libgit2/8fd4f83e8acf5ee813191f08c3433e77200ea38b' (2024-02-22)
  → 'github:libgit2/libgit2/45fd9ed7ae1a9b74b957ef4f337bc3c8b3df01b5' (2023-10-18)

* Cargo.lock: update

* fixup: indexmap deprecation warnings

* Cargo.toml: bump strum to 0.26.1, which to 6.0.0

* CONTRIBUTING: update with more specific commands

CONTRIBUTING.md

@@ -365,9 +365,9 @@ This package uses [Semantic Versioning](https://semver.org/). When determining t
 To cut a release:
 
 * Ensure the `flake.lock`, `Cargo.lock`, and Rust dependencies are up-to-date with the following:
-  + `nix flake update`
-  + `cargo update`
-  + `cargo outdated`
+  + `nix flake update --commit-lock-file`
+  + `cargo outdated --ignore-external-rel --aggressive`
+  + `cargo update --aggressive`
   + Make a PR for for this and let it get merged separately
 * Create a release branch from `main` (`git checkout -b release-v0.0.1`)
 * Remove the `-unreleased` from the `version` field in `Cargo.toml`, `flake.nix`, and the fixture JSON files