Assign default value

Before the unset `DUNECI_OPTS` was used in the next line...
12 jobs for master in 39 minutes and 59 seconds
Status Job ID Name Coverage
  Stage0
passed #526
pull base distributions

00:00:13

 
  Stage1
passed #530
base:16.04

00:00:07

passed #531
base:16.10

00:00:06

passed #528
base:8

00:00:08

passed #529
base:9

00:00:09

passed #527
docker-builder

00:00:08

 
  Stage2
passed #532
base:8-backports

00:00:41

passed #533
dune:2.3

00:00:32

passed #534
dune:2.4

00:00:30

passed #535
dune:git

00:19:56

 
  Stage3
passed #536
dune-fufem:2.4

00:04:04

passed #537
dune-fufem:git

00:15:26