Add a CI job to make sure the Nix Shell is usable

.github/workflows/CI.yml

@@ -142,4 +142,18 @@ jobs:
       - name: Roave BC Check
         uses: docker://nyholm/roave-bc-check-ga
         with:
-          args: roave-backwards-compatibility-check:assert-backwards-compatible --from=v1.0.0
+          args: roave-backwards-compatibility-check:assert-backwards-compatible --from=v1.0.0
+  nix-shell:
+    name: Working Nix Shell
+    runs-on: ubuntu-20.04
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2.4.0
+      - name: Install nix
+        uses: cachix/install-nix-action@v16
+        with:
+          nix_path: nixpkgs=channel:nixos-unstable
+      - name: Install dependencies
+        run: nix-shell --pure --run 'composer install --no-interaction --prefer-dist'
+      - name: Basic tests
+        run: nix-shell --pure --run './vendor/bin/psalm'