fix tar command

overlays/cfg-if-fix.nix

···
 final: prev: {
   buildPackage = args:
-    prev.buildPackage (args // {
-      unpackPhase = ''
-        tar -xzf $src --no-same-owner
-      '';
-    });
-}
+    let
+      pname = args.pname or "";
+    in
+    prev.buildPackage (args // (
+      if pname == "cfg-if" && args.version == "1.0.0" then {
+        unpackPhase = ''
+          tar -xzf $src --no-same-owner
+        '';
+      } else { }
+    ));
+}