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
-''
+''

pkgs/test/stdenv-inputs/default.nix

@@ -4,7 +4,7 @@ let
   foo = stdenv.mkDerivation {
     name = "foo-test";
 
-    unpackPhase = ":";
+    dontUnpack = true;
 
     installPhase = ''
       mkdir -p $out/bin $out/include $out/lib
@@ -22,7 +22,7 @@ let
     name = "bar-test";
     outputs = [ "out" "dev" ];
 
-    unpackPhase = ":";
+    dontUnpack = true;
 
     installPhase = ''
       mkdir -p $out/bin $dev/include $dev/lib