gitlab-ci: Don't rely on $CI_PROJECT_NAME

The name of a forked repository can be changed later, in which case this
would fail to refer to the main repository.

Pointed out by Eric Engestrom in
mesa/mesa!224 .
13 jobs for gitlab-ci-docker-image-only in 3 minutes and 4 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Docker Image
passed #110215
debian-testing

00:00:21

failed #108464
debian-testing

01:00:00

 
  Build
passed #108467
xserver-1.13

00:00:24

passed #108468
xserver-1.14

00:00:25

passed #108469
xserver-1.15

00:00:24

passed #108470
xserver-1.16

00:00:24

passed #108471
xserver-1.17

00:00:25

passed #108472
xserver-1.18

00:00:25

passed #108473
xserver-1.18-clang

00:01:08

passed #108474
xserver-1.19

00:00:24

passed #108475
xserver-1.19-clang

00:00:26

passed #108465
xserver-1.20

00:00:32

passed #108466
xserver-1.20-clang

00:00:31