Fedora 33: reduce concurrency

... to avoid OOM. E.g.:

as: out of memory allocating 4064 bytes after a total of 396447744 bytes
37 jobs for bugfix/f33-oom in 99 minutes and 8 seconds (queued for 15 minutes)
Status Job ID Name Coverage
  Build
passed #77773
docker
build/centos/6

00:11:39

passed #77772
docker
build/centos/7

00:15:20

passed #77771
docker
build/centos/8

00:21:19

passed #77778
docker
build/fedora/29

00:19:26

passed #77777
docker
build/fedora/30

00:18:37

passed #77776
docker
build/fedora/31

00:24:52

passed #77775
docker
build/fedora/32

00:28:23

passed #77864
docker
build/fedora/33

00:26:55

passed #77786
docker
build/opensuse/15.0

00:19:36

passed #77785
docker
build/opensuse/15.1

00:19:49

passed #77784
docker
build/opensuse/15.2

00:17:52

passed #77783
docker
build/sles/12.4

00:24:41

passed #77782
docker
build/sles/12.5

00:29:55

passed #77781
docker
build/sles/15.0

00:24:26

passed #77780
docker
build/sles/15.1

00:20:04

passed #77779
docker
build/sles/15.2

00:23:57

passed #77863
docker
build/fedora/33

00:30:14

passed #77862
docker
build/fedora/33

00:41:18

passed #77840
docker
build/fedora/33

00:36:04

passed #77804
docker
build/fedora/33

00:22:41

passed #77774
docker
build/fedora/33

00:42:03

 
  Test
passed #77789
docker
test/centos/6

00:03:40

passed #77788
docker
test/centos/7

00:03:35

passed #77787
docker
test/centos/8

00:03:36

passed #77794
docker
test/fedora/29

00:04:12

passed #77793
docker
test/fedora/30

00:04:14

passed #77792
docker
test/fedora/31

00:04:53

passed #77791
docker
test/fedora/32

00:04:28

passed #77790
docker
test/fedora/33

00:04:17

passed #77802
docker
test/opensuse/15.0

00:06:17

passed #77801
docker
test/opensuse/15.1

00:06:08

passed #77800
docker
test/opensuse/15.2

00:07:18

passed #77799
docker
test/sles/12.4

00:07:03

passed #77798
docker
test/sles/12.5

00:06:17

passed #77797
docker
test/sles/15.0

00:12:57

passed #77796
docker
test/sles/15.1

00:12:24

passed #77795
docker
test/sles/15.2

00:09:40