Fix and instrument orb setup

[thomashoneyman]

Adds logging to the orb setup and makes it faster.

• Resolve runtime dependencies from the final Nix package set. Previously, the overlay selected uncached coreutils and gzip builds instead of the cached packages provided by nixpkgs. This was already…not the best, but since the recent nixpkgs upgrade, it also causes builds to fail a filesystem-sensitive test in the orb. This is cleaned up now.
• Loads the Nix environment directly so setup stops on Nix errors rather than continuing and failing later with a misleading missing-tool error.
• Preserves the active Nix installation when creating the PATH symlink, including single-user installations.
• Adds stage timings and skips npm audit/funding requests during setup.

[f-f]

👍