ci: cache Nix and Rust build inputs

Reuse Nix store paths, Cargo sources, and Rust target artifacts across ngit-ci runs. Keys track the flake, toolchain, and Cargo dependency inputs while prefix restores preserve useful overlap after updates.

Cache restore and save remain non-fatal optimizations, and the existing formatting, lint, and test commands are unchanged.

Validated by listing the workflow with act and checking the resulting diff.

.ngit/act/workflows/rust_ci.yaml

@@ -22,6 +22,23 @@ jobs:
       - uses: cachix/install-nix-action@v31
         with:
           nix_path: nixpkgs=channel:nixos-unstable
+      - name: Restore Nix store cache
+        continue-on-error: true
+        uses: nix-community/cache-nix-action@v7
+        with:
+          primary-key: nix-${{ runner.os }}-${{ hashFiles('**/*.nix', 'flake.lock') }}
+          restore-prefixes-first-match: nix-${{ runner.os }}-
+      - name: Restore Rust build cache
+        continue-on-error: true
+        uses: actions/cache@v4
+        with:
+          path: |
+            ~/.cargo/git
+            ~/.cargo/registry
+            target
+          key: rust-${{ runner.os }}-${{ hashFiles('**/Cargo.toml', 'Cargo.lock', 'flake.lock') }}
+          restore-keys: |
+            rust-${{ runner.os }}-
       # The suite must be hermetic with respect to ambient git configuration,
       # so CI always runs it under a deliberately hostile global config: a
       # failing pre-commit hook delivered via both core.hooksPath and