electrs: enable unstable build, pin pkg to unstable

stable's buildRustPackage is painfully slow when evaluating (adds >1 sec
even on a fast machine).

This is fixed (NixOS/nixpkgs#69274) in unstable.

pkgs/electrs/default.nix

@@ -1,4 +1,4 @@
-{ lib, rustPlatform, clang, llvmPackages, fetchFromGitHub }:
+{ lib, rustPlatform, clang, llvmPackages, fetchFromGitHub, pkgs }:
 rustPlatform.buildRustPackage rec {
   pname = "electrs";
   version = "0.8.3";
@@ -14,7 +14,12 @@ rustPlatform.buildRustPackage rec {
   buildInputs = [ clang ];
   LIBCLANG_PATH = "${llvmPackages.libclang}/lib";
 
-  cargoSha256 = "19qs8if8fmygv6j74s6iwzm534fybwasjvmzdqcl996xhg75w6gi";
+  cargoSha256 = if pkgs ? cargo-vendor then
+    # nixpkgs ≤ 19.09
+    "19qs8if8fmygv6j74s6iwzm534fybwasjvmzdqcl996xhg75w6gi"
+  else
+    # for recent nixpkgs with cargo-native vendoring (introduced in nixpkgs PR #69274)
+    "1x88zj7p4i7pfb25ch1a54sawgimq16bfcsz1nmzycc8nbwbf493";
 
   meta = with lib; {
     description = "An efficient Electrum Server in Rust";

pkgs/pinned.nix

@@ -1,11 +1,13 @@
 let
   nixpkgsPinned = import ./nixpkgs-pinned.nix;
   unstable = import nixpkgsPinned.nixpkgs-unstable { config = {}; overlays = []; };
+  nixBitcoinPkgsUnstable = import ./. { pkgs = unstable; };
 in
 {
   bitcoin = unstable.bitcoin.override { miniupnpc = null; };
   bitcoind = unstable.bitcoind.override { miniupnpc = null; };
   inherit (unstable)
     clightning
     lnd;
+  inherit (nixBitcoinPkgsUnstable) electrs;
 }