duneci-runner-gc: free some more space

2 jobs for master in 74 minutes and 46 seconds (queued for 1 second)
Status Name Job ID Coverage
  Test
failed docker-builder #16592

01:14:46

failed proxy #16593

01:14:45

 
Name Stage Failure
failed
proxy Test There has been a timeout failure or the job got stuck. Check your timeout limits or try again
Running with gitlab-runner 12.10.1 (ce065b93)
on docker-builder 44cb7da4
Preparing the "docker" executor
Using Docker executor with image duneci/docker-builder ...
Pulling docker image duneci/docker-builder ...
failed
docker-builder Test There has been a timeout failure or the job got stuck. Check your timeout limits or try again
Running with gitlab-runner 12.10.1 (ce065b93)
on docker-builder 44cb7da4
Preparing the "docker" executor
Using Docker executor with image duneci/docker-builder ...
Pulling docker image duneci/docker-builder ...