✨♻️ ci, nix, rust: Set up CI and switch to rust-overlay toolchain

- Added `.github/workflows/test.yaml` to run `cargo test` on push and pull_request events.
- Replaced `fenix` with `rust-overlay` in flake inputs for Rust toolchain management.
- Added `rust-toolchain.toml` specifying stable Rust with required components and targets.
- Updated `flake.nix` to use `rust-bin.fromRustupToolchainFile` and removed `RUST_SRC_PATH`.

.github/workflows/test.yaml

@@ -0,0 +1,15 @@
+name: Test
+
+on:
+  - push
+  - pull_request
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: cachix/install-nix-action@v27
+        with:
+          github_access_token: ${{ secrets.GITHUB_TOKEN }}
+      - run: nix develop -c cargo test