[__letsencrypt_cert] Be more consistent with temp files

Noticed this while doing previous work on the type.

cdist/conf/type/__letsencrypt_cert/gencode-remote

@@ -33,7 +33,7 @@ case "${state}" in
 		fi
 
 		if [ "${certificate_exists}" = "yes" ]; then
-			existing_domains="${__object}/existing_domains"
+			existing_domains=$(mktemp "${TMPDIR:-/tmp}/existing_domains.cdist.XXXXXXXXXX")
 			tail -n +4 "${__object:?}/explorer/certificate-data" | grep -v '^$' > "${existing_domains}"
 			certificate_is_test="$(_explorer_var certificate_is_test)"