refactor: Replace 'depotPath' with 'depot.path'

Instead of having two ways of accessing the path to the depot (one of
which was stuttering, depot.depotPath) we settle on only one:
depot.path.

This was mostly used for NixOS module imports.

Co-Authored-By: Florian Klink <flokli@flokli.de>
Change-Id: I2c0db23383fc34f6ca76baaad4cc4af2d9dfae15
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2962
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>
Reviewed-by: sterni <sternenseemann@systemli.org>

third_party/nix/test-vm.nix

@@ -11,7 +11,7 @@ let
     virtualisation.qemu.options = [ "-nographic" "-curses" ];
 
     nix.nixPath = [
-      "depot=${depot.depotPath}"
+      "depot=${depot.path}"
     ];
   };