fix(automatic-gc): Fix garbage collection script

It needs to refer to this by full path of course.

Change-Id: I911c876ba18877681accb722426314d92b9f2318
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3042
Reviewed-by: lukegb <lukegb@tvl.fyi>
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

ops/modules/automatic-gc.nix

@@ -22,7 +22,7 @@ let
       echo "Have ''${AVAILABLE_KIB} KiB, but want ''${MIN_THRESHOLD_KIB} KiB."
       echo "Triggering Nix garbage collection up to ''${MAX_FREED_BYTES} bytes."
       set -x
-      nix-collect-garbage \
+      ${config.nix.package}/bin/nix-collect-garbage \
         --delete-older-than "''${GEN_THRESHOLD}" \
         --max-freed "''${MAX_FREED_BYTES}"
     else