release.nix: mock rev/lastModified attrs when nixpkgs is a path

Some code-paths expect e.g. `nixpkgs.rev` to exist.
This change makes sure this always exists. This is done on purpose
within this let-in dance rather than in the default such that
`nix-build release.nix --arg nixpkgs ../nixpkgs` also benefits from this
fix.

Change-Id: I94f1f901823d3df433e3c44ae380bb2f67eef70e

release.nix

@@ -4,6 +4,21 @@
   lixSrc ? builtins.fetchGit ./.,
 }:
 
+let
+  nixpkgs' =
+    if builtins.isAttrs nixpkgs then
+      nixpkgs
+    else
+      {
+        outPath = nixpkgs;
+        rev = "0000000000000000000000000000000000000000";
+        shortRev = "0000000";
+        lastModifiedDate = "19700101000000";
+      };
+in
+let
+  nixpkgs = nixpkgs';
+in
 let
   flakeLock = builtins.fromJSON (builtins.readFile (lixSrc + "/flake.lock"));