Build fails immediately with "Cache too full"

Hi, I'm trying to build mesa-git-extension using instructions from https://gitlab.com/freedesktop-sdk/mesa-git-extension I'm on recent arch linux and I've installed buildstream 2.5.0 using `pipx install 'BuildStream == 2.*'`. I have python 3.13.3

Running the command on that page (`make export-repo ARCH=x86_64 REPO=repo`) immediately fails with:
```
bst -o target_arch x86_64 -o bootstrap_arch x86_64 build flatpak-repo.bst
[--:--:--][ ][ main:core activity ] START Build
[--:--:--][ ][ main:core activity ] START Loading elements
[--:--:--][64c76865][ fetch:plugins/buildstream-plugins-community.bst] START mesa-git/plugins-buildstream-plugins-community/64c76865-fetch.20250611-174035.log
[--:--:--][64c76865][ fetch:plugins/buildstream-plugins-community.bst] START Fetching https://files.pythonhosted.org/packages/80/e7/e39284fab4c15cbace51dbcb1896529f0f4a3a1a769a204cf24df8d360e8/buildstream_plugins_community-2.0.2.tar.gz
[00:00:00][64c76865][ fetch:plugins/buildstream-plugins-community.bst] SUCCESS Fetching https://files.pythonhosted.org/packages/80/e7/e39284fab4c15cbace51dbcb1896529f0f4a3a1a769a204cf24df8d360e8/buildstream_plugins_community-2.0.2.tar.gz
[00:00:00][64c76865][ fetch:plugins/buildstream-plugins-community.bst] FAILURE Cache too full
```

The "Cache too full" has me confused. What cache? How big is it? How big does it need to be? I don't think I had anything in the cache before this command - this is the first time I've used buildstream on this machine. The directory I'm building in and ~/.cache are both on an ext4 filesystem with 60 GB free, so I don't think I'm running out of disk space.

If this isn't a bug, then at the very least, this error message should provide enough information that I could do something about it. At the moment, I have no idea if I need more free disk space or if I need to configure bst differently or what.

Start by reproducing `make export-repo ARCH=x86_64 REPO=repo` and the `bst ... build flatpak-repo.bst` command from the report, then trace the `Cache too full` failure after fetching `buildstream_plugins_community-2.0.2.tar.gz`. Done means the error identifies the relevant cache, its size or limit, and the configuration or remediation needed to proceed.

Written by the indexing model from the issue text.