gitlab-ci: use a stable build directory
This should allow for sccache to get better hit rates with its builds.
@robertmaynard @utkarsh.ayachit Trying this out here. This should allow for better cache hits for MRs on the same project at least. Note that this variable is ignored for non-container runs (docs say docker-windows
also ignores it, but I wonder if that's an oversight in the docs or the code), so shell-based runners will still need some runner coordination (usernames basically).