ed: avoid the useless rebuild due to #21752

pkgs/applications/editors/ed/default.nix

@@ -2,7 +2,6 @@
 
 stdenv.mkDerivation rec {
   name = "ed-1.13";
-  file_md5 = "fb8ffc8d8072e13dd5799131e889bfa5"; # for fedora mirror
 
   src = fetchurl {
     # gnu only provides *.lz tarball, which is unfriendly for stdenv bootstrapping
@@ -10,10 +9,12 @@ stdenv.mkDerivation rec {
     # When updating, please make sure the sources pulled match those upstream by
     # Unpacking both tarballs and running `find . -type f -exec sha256sum \{\} \; | sha256sum`
     # in the resulting directory
-    urls = [
-      "http://pkgs.fedoraproject.org/repo/extras/ed/${name}.tar.bz2/${file_md5}/${name}.tar.bz2"
-      "http://fossies.org/linux/privat/${name}.tar.bz2"
-    ];
+    urls = let file_md5 = "fb8ffc8d8072e13dd5799131e889bfa5"; # for fedora mirror
+      in [
+        ("http://pkgs.fedoraproject.org/repo/extras/ed"
+          + "/${name}.tar.bz2/${file_md5}/${name}.tar.bz2")
+        "http://fossies.org/linux/privat/${name}.tar.bz2"
+      ];
     sha256 = "1iym2fsamxr886l3sz8lqzgf00bip5cr0aly8jp04f89kf5mvl0j";
   };