setup_dxvk.sh: install with reflinks if supported

Some filesystems, such as XFS and BTRFS, may perform a lightweight copy
with CoW. This is most useful for rarely changing content, and the
usecase of installing DXVK is one of such usecases, as library files are
usually not modified (might be removed, but it's not modification).

So make use of it whenever supported.

setup_dxvk.sh

@@ -25,7 +25,7 @@ esac
 shift
 
 with_dxgi=true
-file_cmd="cp -v"
+file_cmd="cp -v --reflink=auto"
 
 while (($# > 0)); do
   case "$1" in