treewide: use dontUnpack

pkgs/test/patch-shebangs/default.nix

@@ -3,7 +3,7 @@
 let
   bad-shebang = stdenv.mkDerivation {
     name         = "bad-shebang";
-    unpackPhase  = ":";
+    dontUnpack = true;
     installPhase = ''
       mkdir -p $out/bin
       echo "#!/bin/sh" > $out/bin/test
@@ -23,4 +23,4 @@ in runCommand "patch-shebangs-test" {
     echo "no" >&2
     exit 1
   fi
-''
+''