flake.nix, flake.lock: bump nixpkgs and pinned tools

nixpkgs: bump unstable revision. Pinned tools: grpc-gateway 2.28.0 ->
2.29.0 (matches the Go dep), golangci-lint 2.11.4 -> 2.12.2. Disable
gomodguard in .golangci.yaml since 2.12 deprecated it.