Be very clear about which path we want modified

ci-tools/update-nixpkgs

@@ -59,7 +59,7 @@ nix-build -A morph -o result-before
 
 # Spawn *another* nix-shell that has the *other* update-nixpkgs tool.  Should
 # sort out this mess sooner rather than later...
-nix-shell ../shell.nix --run 'update-nixpkgs ./nixpkgs.json'
+nix-shell ../shell.nix --run 'update-nixpkgs ${PWD}/nixpkgs.json'
 
 # Show us what we did
 if git diff --exit-code; then