refactor(build): Pin dependencies using Go modules

Drops the go2nix configuration in favour of pkgs.buildGoModule.

Note that the go.sum file is bloated by issues with cyclic
dependencies in some Google projects, but this large number of
dependencies is not actually built.

tools/nixery/.github/workflows/build-and-test.yaml

@@ -20,6 +20,6 @@ jobs:
       - name: Check formatting
         run: "test -z $(gofmt -l .)"
       - name: Build Nixery
-        run: "nix-build --arg maxLayers 2 --no-out-link"
+        run: "nix-build --no-out-link"
       - name: Run integration test
         run: scripts/integration-test.sh