treewide: preserve hashes

avoid mass rebuild from the last commits

pkgs/development/libraries/boost/generic.nix

@@ -101,6 +101,8 @@ stdenv.mkDerivation {
 
   inherit src;
 
+  patchFlags = "";
+
   patches = patches
     ++ optional stdenv.isDarwin ./darwin-no-system-python.patch;
 

pkgs/tools/compression/brotli/default.nix

@@ -15,6 +15,8 @@ stdenv.mkDerivation rec {
 
   nativeBuildInputs = [ cmake ];
 
+  cmakeFlags = [];
+
   outputs = [ "out" "dev" "lib" ];
 
   doCheck = true;

pkgs/tools/text/html-tidy/default.nix

@@ -13,6 +13,8 @@ stdenv.mkDerivation rec {
 
   nativeBuildInputs = [ cmake libxslt/*manpage*/ ];
 
+  cmakeFlags = [];
+
   # ATM bin/tidy is statically linked, as upstream provides no other option yet.
   # https://github.com/htacg/tidy-html5/issues/326#issuecomment-160322107