fix(ops/nixos): use builtins.storePath to avoid dumping pkgs.path

This is a less invasive way to achieve the same goal as cl/5681, by
preventing the already existing nixpkgs store path from being dumped
again at the call site. To support nixpkgsBisectPath, we simply check if
pkgs.path is below builtins.storeDir and use builtins.storePath based on
that.

This is actually similar to the approach taken in the nixpkgs
documentation system which tries to limit the amount of nixpkgs that
needs to be dumped by using filterSource on specific subtrees of
nixpkgs. For this to work it has to insist on pkgs.path being an
ordinary Nix path, though.

Change-Id: Idf892f90a5d811184568e4702a901c334d56210e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/5787
Autosubmit: sterni <sternenseemann@systemli.org>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

ops/nixos.nix

@@ -7,10 +7,18 @@ in rec {
   baseModule = { ... }: {
     # Ensure that pkgs == third_party.nix
     nixpkgs.pkgs = depot.third_party.nixpkgs;
-    nix.nixPath = [
-      ("nixos=" + pkgs.path)
-      ("nixpkgs=" + pkgs.path)
-    ];
+    nix.nixPath =
+      let
+        # Due to nixpkgsBisectPath, pkgs.path is not always in the nix store
+        nixpkgsStorePath =
+          if lib.hasPrefix builtins.storeDir (toString pkgs.path)
+          then builtins.storePath pkgs.path # nixpkgs is already in the store
+          else pkgs.path; # we need to dump nixpkgs to the store either way
+      in
+      [
+        ("nixos=" + nixpkgsStorePath)
+        ("nixpkgs=" + nixpkgsStorePath)
+      ];
   };
 
   nixosFor = configuration: (depot.third_party.nixos {