stdenv/setup.sh: make substitute* not drop EOF newline

This should solve #1982.

pkgs/stdenv/generic/setup.sh

@@ -304,9 +304,11 @@ substitute() {
 
     local -a params=("$@")
 
-    local n p pattern replacement varName
+    local n p pattern replacement varName content
 
-    local content="$(cat $input)"
+    # a slightly hacky way to keep newline at the end
+    content="$(cat $input; echo -n X)"
+    content="${content%X}"
 
     for ((n = 2; n < ${#params[*]}; n += 1)); do
         p=${params[$n]}