Add nix test.

.github/workflows/nix_tests.yaml

@@ -0,0 +1,32 @@
+name: "Nix Tests"
+on:
+  pull_request:
+    paths:
+      - ".github/workflows/tests.yaml"
+      - "server/**"
+      - "proto/**"
+      - "router/**"
+      - "launcher/**"
+      - "Cargo.lock"
+      - "rust-toolchain.toml"
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}
+  cancel-in-progress: true
+
+jobs:
+  tests:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v4
+    - uses: cachix/install-nix-action@v27
+      with:
+        nix_path: nixpkgs=channel:nixos-unstable
+    - uses: cachix/cachix-action@v14
+      with:
+        name: text-generation-inference
+        # If you chose signing key for write access
+        signingKey: '${{ secrets.CACHIX_SIGNING_KEY }}'
+    - name: Build
+      run: nix build
+    - name: Run tests.
+      run: nix develop .#impure --command python -m pytest server/tests/