test: use nixpkgs-unstable flake input for nix-bitcoin pkgs
By using the `default` flake module for tests, `pkgsUnstable` in `pkgs/default.nix` gets passed the `nixpkgs-unstable` flake input instead of falling back to importing `nixpkgsPinned.nixpkgs-unstable`. For some use cases this prevents importing `nixpkgs-unstable` twice (once via the `nixpkgs-unstable` flake input, once via `import nixpkgsPinned.nixpkgs-unstable`).
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