treewide: use less phases if not necessary

This removes some skipping of e.g. fixupPhase and cleans up occurences
where this led to duplicating code

pkgs/tools/networking/quicktun/default.nix

@@ -13,14 +13,11 @@ stdenv.mkDerivation rec {
 
   buildInputs = [ libsodium ];
 
-  phases = [ "unpackPhase" "buildPhase" "installPhase" ];
-
   buildPhase = "bash build.sh";
 
   installPhase = ''
-    mkdir -p $out/bin
     rm out/quicktun*tgz
-    cp -v out/quicktun* $out/bin/
+    install -vD out/quicktun* -t $out/bin
   '';
 
   meta = with stdenv.lib; {