Install the image's Ubuntu packages from the normal mirrors (closes #190)

The final image stage now apt-gets ca-certificates, nix-bin and runit
from Ubuntu's archive, with no snapshot and no hash check of the package
lists. The borrowed CA bundle is dropped: apt fetches the packages over
plain HTTP and checks each one against the archive's signed package
lists, so the install needs none. The Ubuntu base image stays pinned by
digest, and every other pin is kept. README.md, SPEC.md and
docs/image.md no longer describe a snapshot.

Model: deepseek-v4-pro (implementation), opus-5-5 (rework)

docs/image.md

@@ -8,8 +8,7 @@ is pinned that way.
 ## Contents
 
 - Ubuntu 26.04 LTS, pinned by digest.
-- `ca-certificates`, `nix-bin` and `runit` from Ubuntu, from the snapshot of its
-  archive at `20261001T000000Z`.
+- `ca-certificates`, `nix-bin` and `runit` from Ubuntu's archive.
 - Nix, with nixpkgs `nixos-26.05.11045.774debe7a0d1` set up for root as
   `nixpkgs`. No Nix daemon; `build-users-group =` in `/etc/nix/nix.conf`. Root's
   Nix profile, `/nix/var/nix/profiles/default/bin`, last on the `PATH`.
@@ -26,10 +25,8 @@ is pinned that way.
 
 ## Pins
 
-- Ubuntu's packages: the snapshot service, `--snapshot 20261001T000000Z` on both
-  `apt-get update` and `apt-get install`. The SHA-256 hash of each of the
-  snapshot's `InRelease` files is named in the `Dockerfile` and checked after
-  `apt-get update`. The hashes are the amd64 archive's.
+- Ubuntu's packages are not pinned: they come from the live archive, whose
+  packages change with every update.
 - nixpkgs: the release file `nixexprs.tar.xz` of
   `nixos-26.05.11045.774debe7a0d1` on `releases.nixos.org`, named with its
   SHA-256 hash.