Reduce space requirements for GitLab CI
Another pipeline failed because the CI runner ran out of space. Quoting @grote:
yeah the server is out of space again, this was quick
There's a runner caches with 21 GB, 15 GB, 11 GB, etc.
The 21 GB one is for briar-desktop where each branch takes up more than 642M, the nightly builds 1.3GB