use unstable alias for astroid

roles/mail.nix

@@ -1,10 +1,11 @@
 { config, lib, pkgs, ... }:
 
 let
+  unstable = import <nixos-unstable> { config.allowUnfree = true; };
   notcoal = pkgs.callPackage (import ../packages/notcoal) {};
 in {
   environment.systemPackages = with pkgs; [
-    astroid
+    unstable.astroid
     isync
     khal
     msmtp

roles/work.nix

@@ -31,7 +31,6 @@ in {
     cloudstation
     nagstamon
 
-    astroid
     drone-cli
     go-jira
     govc