duneci-standard-test: use /duneci/opts.gcc by default

Otherwise no opts file is used at all and the code is built without any
optimization, leading to timeouts in tests.

base-common/duneci-standard-test

@@ -3,7 +3,7 @@
 set -e
 set -u
 
-if [[ -n "${DUNECI_OPTS:-}" ]]; then
+if [[ -n "${DUNECI_OPTS:-/duneci/opts.gcc}" ]]; then
   set -- --opts="${DUNECI_OPTS}" "${@}"
 fi