feat(ops/modules): Provide some modules to all nixoses

For modules that are gated behind a mkEnableOption, it's reasonable to
just provide them to all Depot-built nixos systems without requiring
people to explicitly import them. This defines a special module called
`default-imports.nix` which imports these modules (currently just
tvl-cache.nix and automatic-gc.nix, as I'm being rather conservative
adding things here to avoid breaking anyone's system), then provides
that module as one of the `modules` passed at the top-level
nixos/eval-config invocation.

Change-Id: I3be299ab10ae4c451ef11c514edb3c89318a2278
Reviewed-on: https://cl.tvl.fyi/c/depot/+/4345
Tested-by: BuildkiteCI
Autosubmit: grfn <grfn@gws.fyi>
Reviewed-by: tazjin <mail@tazj.in>

third_party/default.nix

@@ -12,7 +12,7 @@
 #    other folders below //third_party, other than the ones mentioned
 #    above.
 
-{ pkgs, ... }:
+{ pkgs, depot, ... }:
 
 {
   # Expose a partially applied NixOS, expecting an attribute set with
@@ -33,7 +33,10 @@
   let
     eval = import "${pkgs.path}/nixos/lib/eval-config.nix" {
       inherit specialArgs system;
-      modules = [ configuration ];
+      modules = [
+        configuration
+        (import "${depot.path + "/ops/modules/default-imports.nix"}")
+      ];
     };
 
     # This is for `nixos-rebuild build-vm'.