fix: use localSystem over builtins.currentSystem where appropriate

cl/5832 added a global system parameter to depot which allowed specifying
what `system` should be used for nixpkgs and all depot derivations
(assuming a native compilation case) which was implemented in cl/5846.
This allows instantiating derivations for a different system than
whatever builtins.currentSystem happens to be. This is useful for
debugging, allows you to schedule builds on build servers for
other platforms or build for architectures that are a subset of
the one you are running (e.g. i686-linux).

This change eliminates all remaining uses of builtins.currentSystem
which could lead to an inconsistent combination of `system` values
when passing `localSystem`.

Change-Id: I0f824f4f0afa88ef1ddd9a8cecb24bf94bacde7a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7260
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>
Autosubmit: sterni <sternenseemann@systemli.org>

third_party/default.nix

@@ -12,7 +12,7 @@
 #    other folders below //third_party, other than the ones mentioned
 #    above.
 
-{ pkgs, depot, ... }:
+{ pkgs, depot, localSystem, ... }:
 
 {
   # Expose a partially applied NixOS, expecting an attribute set with
@@ -27,7 +27,7 @@
   nixos =
     { configuration
     , specialArgs ? { }
-    , system ? builtins.currentSystem
+    , system ? localSystem
     , ...
     }:
     let