ci/cache: Prune profile generations

Explicitly remove expired generations from the nonstandard Harmonia
profile before collecting garbage. This keeps old CI closures from
remaining rooted until the cache exhausts its inodes.

ci/push-to-cache.sh

@@ -12,7 +12,10 @@ remote_cmd() {
 
 collect_garbage() {
   echo "Collecting garbage..."
-  remote_cmd nix-collect-garbage --delete-older-than 60d
+  remote_cmd nix-env \
+    -p "$REMOTE_STORE"/nix/var/nix/profiles/nixfiles \
+    --delete-generations 60d
+  remote_cmd nix-collect-garbage
 }
 
 umask_old=$(umask)

docs/deployment.md

@@ -223,8 +223,8 @@ Harmonia substituter as the boxes), runs `nix flake check --no-build`, then buil
 of `.#ci.x86_64-linux`: systems as `system-<name>`, homes as `home-<name>` (with `@` changed to
 `-at-`), packages as `package-<name>`, and the development `shell`. Each result is pushed to the
 Harmonia cache with [`ci/push-to-cache.sh`](../ci/push-to-cache.sh). Before the first push, the
-workflow collects cache paths older than its retention period so a full cache cannot prevent
+workflow deletes cache-profile generations older than its retention period, then collects
-collection from being reached.
+unreachable paths so a full cache cannot prevent collection from being reached.
 
 It then builds `.#ciDrv.x86_64-linux`, a `linkFarm` of all CI attributes, and pushes it with
 `UPDATE_PROFILE=1`. That updates the `nixfiles` profile on the cache box and collects old paths