treewide: use dontUnpack

pkgs/tools/graphics/eplot/default.nix

@@ -17,7 +17,7 @@ stdenv.mkDerivation rec {
 
   buildInputs = [ ruby ];
 
-  unpackPhase = "true";
+  dontUnpack = true;
 
   installPhase = ''
     mkdir -p "$out/bin"

pkgs/tools/graphics/zxing/default.nix

@@ -12,7 +12,7 @@ stdenv.mkDerivation rec {
     sha256 = "0fzxvvf5dqyrs5m9rqw4ffm9h1s27bi7q3jb1dam34s80q2rp2zq";
   };
   inherit jre;
-  unpackPhase = ''true'';
+  dontUnpack = true;
   installPhase = ''
     mkdir -p "$out/lib/java" "$out/bin"
     cp "${core_jar}" "${javase_jar}" "$out/lib/java"