fix: use depsTargetTargetPropagated for depot's gcroot

We've been seeing CI failures after a recent nixpkgs bump which only
occur on canon *after* a CL has been merged. In these CI failures, the
`ci.gcroot` attribute is built to "anchor" the latest canon build, but
fails because it tries to execute random non-executable files (e.g.
the sqlite database produced by //corp/russian/data-import).

From what I can tell, there has been a recent change in nixpkgs where
makeSetupHooks' arguments were restructured. What was previously
`deps` is now `propagatedBuildInputs`, and
`depsTargetTargetPropagated`.

The latter is supposed to be used for non-executable targets,
according to the docs.

This commit changes the entire set to use that flag, as we don't
actually want any executable setup hook - just a gcroot.

Change-Id: Ib258c4f3b09d746c3d000d1c7480d2b4101ab75e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8227
Autosubmit: tazjin <tazjin@tvl.su>
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

default.nix

@@ -118,7 +118,7 @@ readTree.fix (self: (readDepot {
   ci.gcroot = with self.third_party.nixpkgs; makeSetupHook
     {
       name = "depot-gcroot";
-      deps = self.ci.targets;
+      depsTargetTargetPropagated = self.ci.targets;
     }
     emptyFile;
 })