Only addwrite for tmp dirs if sandbox is enabled.

This should fix bug #307729.
diff --git a/bin/ebuild.sh b/bin/ebuild.sh
index 0d59f4e..42ca1fe 100755
--- a/bin/ebuild.sh
+++ b/bin/ebuild.sh
@@ -106,7 +106,7 @@
 	export TEMP=$T
 	export TMP=$T
 	export TMPDIR=$T
-else
+elif [[ $SANDBOX_ON = 1 ]] ; then
 	for x in TEMP TMP TMPDIR ; do
 		[[ -n ${!x} ]] && addwrite "${!x}"
 	done