scripts/install-nix-from-closure: only show progress if a terminal is used

While the progress dots during the copying of the store work fine on a
normal terminal, those look pretty off if the script is run inside a
provisioning script of e.g. `vagrant` or `packer` where `stderr` and
`stdout` are captured:

    default: .
    default: ..
    default: .
    default: .
    default: .

To work around this, the script checks with `-t 0` if it's
running on an actual terminal and doesn't show the progress if that's not
the case.

scripts/install-nix-from-closure.sh

@@ -166,9 +166,15 @@ fi
 mkdir -p $dest/store
 
 printf "copying Nix to %s..." "${dest}/store" >&2
+# Insert a newline if no progress is shown.
+if [ ! -t 0 ]; then
+  echo ""
+fi
 
 for i in $(cd "$self/store" >/dev/null && echo ./*); do
+    if [ -t 0 ]; then
       printf "." >&2
+    fi
     i_tmp="$dest/store/$i.$$"
     if [ -e "$i_tmp" ]; then
         rm -rf "$i_tmp"