tsic, gh: keep unstable on Docker Hub

ghcr.io/tailscale/tailscale:unstable is stale (last updated 2022,
points to v1.35.25). Only tailscale/tailscale on Docker Hub publishes
current unstable builds, so tsic.go reverts the unstable case and
build-tailscale-released pulls unstable from Docker Hub while keeping
release tags on ghcr.io.

Release tags on ghcr.io match Docker Hub by digest (verified v1.96),
so the rate-limit avoidance for the bulk of pulls is preserved.

Add the conditional docker/login-action step to the new job so the
main repo gets authenticated pulls; fork PRs fall through to anonymous
DH for the single unstable pull per CI run, well under the 100/6h
anonymous limit.