build(nix): add mold to the flake dev shell (#2122)

.github/workflows/nix.yml

@@ -6,12 +6,14 @@ on:
     paths:
       - "**/*.nix"
       - "flake.lock"
+      - "rust-toolchain.toml"
   pull_request:
     branches: ["main", "develop"]
     types: [opened, synchronize, reopened, ready_for_review]
     paths:
       - "**/*.nix"
       - "flake.lock"
+      - "rust-toolchain.toml"
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
@@ -32,5 +34,11 @@ jobs:
       - name: Magic Nix Cache
         uses: DeterminateSystems/magic-nix-cache-action@v6
 
-      - name: Check full devShell
+      - name: Warm up full devShell
         run: nix develop .#full --command true
+
+      - name: Cargo check in flake environment
+        run: nix develop .#full --command cargo check
+
+      - name: Cargo build in flake environment
+        run: nix develop .#full --command cargo build