flake.nix, go.mod: bump tailscale.com to v1.97.0-pre

Pulls in the cmd/nardump library split (tailscale/tailscale#19551)
so flakehashes.json tooling can import nardump.SRI directly.

Side effects: Go directive bumps to 1.26.2 and the nixpkgs lock
advances to a revision shipping go 1.26.2.