nix-prefetch-zip: keep downloaded file extension

Keep the correct file extension on the downloaded file so that
`unpackFile` will know how to unpack it correctly.

pkgs/build-support/fetchzip/nix-prefetch-zip

@@ -73,7 +73,7 @@ mkdir -p $unpackDirTmp
 unpackDir=$tmp/unpacked/$name
 mkdir -p $unpackDir
 
-downloadedFile=$tmp/$name
+downloadedFile=$tmp/$(basename "$url")
 
 unpackFile() {
   local curSrc="$1"