Add flake.nix for installation on NixOS

[mikl]

Add a flake.nix file, as described by Clåde:

The flake.nix is complete, builds successfully, and passes nix flake check. Here's what it provides:

Package (packages.ldddns / packages.default):

• Builds the Go binary with buildGo126Module, CGO disabled, -trimpath, and stripped symbols
• Sets main.version via ldflags from the git revision
• Renames the binary from ldddns.arnested.dk (the Go module name) to ldddns
• vendorHash is computed from the current go.sum

NixOS module (nixosModules.default):

• services.ldddns.enable - enables the systemd service
• services.ldddns.package - override the package
• services.ldddns.hostnameLookup - configures hostname lookup methods (defaults to ["env:VIRTUAL_HOST" "containerName"])
• services.ldddns.ignoreDockerComposeOneoff - ignore oneoff containers (default true)
• services.ldddns.gops - enable gops diagnostics (default false)
• The systemd unit mirrors the hardening from systemd/ldddns.service

Dev shell (devShells.default):

• Go 1.26, gopls, and goreleaser

Usage in a NixOS configuration:

{
  inputs.ldddns.url = "github:arnested/ldddns";

  outputs = { nixpkgs, ldddns, ... }: {
    nixosConfigurations.myhost = nixpkgs.lib.nixosSystem {
      modules = [
        ldddns.nixosModules.default
        { services.ldddns.enable = true; }
      ];
    };
  };
}

Notes:

• The prePatch relaxes the go 1.26.3 directive to go 1.26 because nixpkgs-unstable currently ships Go 1.26.2. This can be removed once nixpkgs catches up.
• Tests are disabled (doCheck = false) because honnef.co/go/netdb reads /etc/protocols at init time, which doesn't exist in the Nix sandbox.
• The vendorHash will need updating whenever go.mod/go.sum changes.