ci: bring back Coverity

Looks like DOCKER_EXEC got lost somewhere along the way, which, in
turn, caused the "coverity" job to fail with
```
$ $DOCKER_EXEC meson cov-build -Dman=false
Command 'meson' not found, but can be installed with:
apt install meson
Please ask your administrator.
```

https://travis-ci.org/github/systemd/systemd/builds/701705788