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 (https://github.com/NixOS/nixpkgs/pull/69274) in unstable.

pkgs/pinned.nix

@@ -1,6 +1,7 @@
 let
   nixpkgsPinned = import ./nixpkgs-pinned.nix;
   unstable = import nixpkgsPinned.nixpkgs-unstable { config = {}; overlays = []; };
+  nixBitcoinPkgsUnstable = import ./. { pkgs = unstable; };
 in
 {
   bitcoin = unstable.bitcoin.override { miniupnpc = null; };
@@ -8,4 +9,5 @@ in
   inherit (unstable)
     clightning
     lnd;
+  inherit (nixBitcoinPkgsUnstable) electrs;
 }