stdenv: don't clobber useArray and type in {prepend,append}ToVar

Some other packages, for example ruby gems via buildRubyGem, use a
variable called "type" internally, which is overwritten here and
causes failures like:

    failure: $gempkg path unspecified

Fix for changes in 11c3127e38.

pkgs/stdenv/generic/setup.sh

@@ -206,8 +206,8 @@ addToSearchPath() {
 # syntax when they switch to setting __structuredAttrs = true.
 prependToVar() {
     local -n nameref="$1"
+    local useArray type
 
-    useArray=
     if [ -n "$__structuredAttrs" ]; then
         useArray=true
     else
@@ -239,8 +239,8 @@ prependToVar() {
 # Same as above
 appendToVar() {
     local -n nameref="$1"
+    local useArray type
 
-    useArray=
     if [ -n "$__structuredAttrs" ]; then
         useArray=true
     else