treewide: *Flags convert to list from str

*Flags implies a list

slightly relevant:
> stdenv: start deprecating non-list configureFlags https://github.com/NixOS/nixpkgs/pull/173172

the makeInstalledTests function in `nixos/tests/installed-tests/default.nix` isn't available outside of nixpkgs so
it's not a breaking change

pkgs/development/interpreters/python-cosmopolitan/default.nix

@@ -9,7 +9,7 @@ stdenv.mkDerivation rec {
   nativeBuildInputs = [ bintools-unwrapped unzip ];
 
   # slashes are significant because upstream uses o/$(MODE)/foo.o
-  buildFlags = "o//third_party/python";
+  buildFlags = [ "o//third_party/python" ];
   checkTarget = "o//third_party/python/test";
   enableParallelBuilding = true;