use unstable flutter thru channel

.drone.yml

@@ -9,5 +9,5 @@ steps:
       #- nix-shell --pure --run "dart --version"
       - nix-shell --pure --run "flutter build apk --split-per-abi"
     environment:
-      NIX_PATH: "nixpkgs=/nix/var/nix/profiles/per-user/root/channels/nixpkgs"
+      NIX_PATH: "nixpkgs=/nix/var/nix/profiles/per-user/root/channels/nixos"
       NIXPKGS_ALLOW_UNFREE: 1

shell.nix

@@ -1,6 +1,8 @@
 { pkgs ? import <nixpkgs> { config.android_sdk.accept_license = true; } }:
 
 let
+  unstable = import (fetchTarball "https://nixos.org/channels/nixpkgs-unstable/nixexprs.tar.xz") {};
+
   androidComposition = pkgs.androidenv.composeAndroidPackages {
     toolsVersion = "26.1.1";
     platformToolsVersion = "33.0.2";
@@ -10,5 +12,5 @@ in
 pkgs.mkShell {
   ANDROID_HOME= "${androidComposition.androidsdk}/libexec/android-sdk";
   ANDROID_SDK_ROOT = "${androidComposition.androidsdk}/libexec/android-sdk";
-  nativeBuildInputs = with pkgs; [ flutter openjdk11_headless ];
+  nativeBuildInputs = with pkgs; [ unstable.flutter openjdk11_headless ];
 }