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

.travis.yml

@@ -47,6 +47,7 @@ jobs:
       env:
         - FEDORA_RELEASE="latest"
         - TOOL_BASE="/var/tmp/coverity-scan-analysis"
+        - DOCKER_EXEC="docker exec -ti $CONT_NAME"
         - DOCKER_RUN="docker run -v $TOOL_BASE:$TOOL_BASE:rw --env-file .cov-env"
           # Coverity env variables
         - PLATFORM="$(uname)"