Hacked-together approach to collecting garbage from local flat-file binary caches.
Build the package, and run bin/cache-gc with the path to the binary
cache. It will compute the closures of all the paths in the cache, and
compute which paths can be deleted (are not referenced by any paths
newer than --days, 90 by default):
result/bin/cache-gc --days 30 /var/lib/hydra/cache
For automated use, --delete can be passed, which does not require
confirmation before actually deleting files. Use with care!