GC pinning

[srid]

Enable garbage collection, but first allow pinning certain builds (like master branch of repos).

Pretty much this idea but for Jenkins: hercules-ci/hercules-ci-agent#482

The nixCI feature can support this, for instance:

nixCI pinGcRoot: true

(Uses ${jobName}-${branchName} as name; overwriting previous pinned roots if any)

---

We are not far from exhausting free disk space on the CI machine,

[admin@jenkins-nix-ci:~]$ df -h /
Filesystem                                              Size  Used Avail Use% Mounted on
/dev/disk/by-uuid/63a89c69-bd66-45ff-9b97-5bb406424fa5  1.9T  1.2T  618G  66% /

[srid]

Also consider this from the context of cachix (or any nix cache): https://docs.cachix.org/pins