remove test matrix

`nix build` already performs `cargo test`

.github/workflows/build.yml

@@ -28,37 +28,9 @@ jobs:
 
       - run: nix flake check
 
+        # this also includes `cargo test`
       - name: build statix
         run: nix build -L
 
       - name: build and install statix with flake-compat
         run: nix-env -if default.nix
-
-  tests:
-    strategy:
-      matrix:
-        os:
-          - ubuntu-latest
-        nix:
-          - name: 2.5.1
-            url: https://releases.nixos.org/nix/nix-2.5.1/install
-    runs-on: ${{ matrix.os }}
-    steps:
-      - uses: actions/checkout@v2.3.5
-
-      - uses: cachix/install-nix-action@v15
-        with:
-          install_url: ${{ matrix.nix.url }}
-          extra_nix_config: |
-            experimental-features = nix-command flakes
-
-      - uses: cachix/cachix-action@v10
-        with:
-          name: statix
-          authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
-
-      - name: set up pinned nixpkgs
-        run: echo "NIX_PATH=nixpkgs=$(nix-instantiate --eval flake-compat.nix -A defaultNix.inputs.nixpkgs.outPath | sed 's|\"||g')" >> $GITHUB_ENV
-
-      - run: nix-shell --run "cargo test"
-