fix: make depot-gcroot derivation a plain list of out paths

This just creates a text file (similar to the propagated deps file)
which lists the outpaths of all targets.

It's unclear why the previous fix didn't work, but it was done solely
based on docs not on reading code. In general we don't really need a
setup hook though and nobody remembers why that was the solution we
used anyways, so lets remove it.

Change-Id: I2a945925f4fa56b272aa44c29229b6a1aefa80e6
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8228
Reviewed-by: flokli <flokli@flokli.de>
Autosubmit: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

default.nix

@@ -115,10 +115,9 @@ readTree.fix (self: (readDepot {
   });
 
   # Derivation that gcroots all depot targets.
-  ci.gcroot = with self.third_party.nixpkgs; makeSetupHook
-    {
-      name = "depot-gcroot";
-      depsTargetTargetPropagated = self.ci.targets;
-    }
-    emptyFile;
+  ci.gcroot = with self.third_party.nixpkgs; writeText "depot-gcroot"
+    (builtins.concatStringsSep "\n"
+      (lib.flatten
+        (map (p: map (o: p.${o}) p.outputs or [ ]) # list all outputs of each drv
+          self.ci.targets)));
 })