update github ci to use flake

.github/workflows/nix.yml

@@ -1,5 +1,5 @@
 ---
-name: nix
+name: Test Nix
 on: [push, pull_request]
 jobs:
   test:
@@ -11,12 +11,23 @@ jobs:
       run:
         working-directory: ./
     steps:
-      - uses: actions/checkout@v2.3.4
-      - uses: cachix/install-nix-action@v14.1
+      - uses: actions/checkout@v3
+      - uses: cachix/install-nix-action@v20
         with:
           nix_path: nixpkgs=channel:nixos-unstable
           name: fipy
           authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
           extraPullNames: nix-community
-      - run: nix-shell --pure --command "python setup.py test"
-      - run: nix-shell --pure --command "py.test --nbval --sanitize-with examples/sanitize.cfg examples/index.ipynb"
+      - run: echo "UID=${UID}" >> $GITHUB_ENV
+      - uses: actions/cache/restore@v3
+        with:
+          path: /run/user/${{ env.UID }}/http_cache.sqlite
+          key: ${{ runner.os}}-http_cache.sqlite
+      - run: nix develop --command bash -c "python setup.py test"
+      - run: nix develop --command bash -c "py.test --nbval --sanitize-with examples/sanitize.cfg examples/index.ipynb"
+      - uses: actions/cache/save@v3
+        if: always()
+        with:
+          path: /run/user/${{ env.UID }}/http_cache.sqlite
+          key: ${{ runner.os}}-http_cache.sqlite
+