Add flake.nix

NixOS cannot execute the generic Linux `electrs` binary that the
`electrsd` crate downloads for the integration tests. Provide a devshell
that supplies bitcoind and a source-built Blockstream electrs from
nixpkgs, along with `BITCOIND_EXE` and `ELECTRS_EXE` environment
variables.

Author: rustaceanrob

flake.lock

@@ -0,0 +1,40 @@
+{
+  description = "ldk-node development environment";
+
+  inputs = {
+    # Pinned to nixos-25.05: newer revisions of nixpkgs ship a
+    # blockstream-electrs whose CLI moved from `--cookie-file` to `--cookie`,
+    # This is a break in the `electrs` crate.
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-25.05";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs = { self, nixpkgs, flake-utils }:
+    flake-utils.lib.eachDefaultSystem (system:
+      let
+        pkgs = import nixpkgs {
+          inherit system;
+          # `blockstream-electrs` in this nixpkgs revision transitively pulls
+          # in `python3.12-ecdsa-0.19.1`, which nixpkgs marks insecure.
+          config.permittedInsecurePackages = [ "python3.12-ecdsa-0.19.1" ];
+        };
+
+        # Upstream tests are failing. Skips the tests on install.
+        blockstream-electrs = pkgs.blockstream-electrs.overrideAttrs (_: {
+          doCheck = false;
+        });
+      in
+      {
+        devShells.default = pkgs.mkShell {
+          packages = [
+            pkgs.bitcoind
+            blockstream-electrs
+          ];
+
+          shellHook = ''
+            export BITCOIND_EXE="${pkgs.bitcoind}/bin/bitcoind"
+            export ELECTRS_EXE="${blockstream-electrs}/bin/electrs"
+          '';
+        };
+      });
+}