Nightly

tests: add RFC 6793 two-octet-AS e2e interop test against real BIRD

Unit tests already cover the AS4_PATH/AS4_AGGREGATOR reconcile and
downgrade logic byte-exactly against GoBGP-generated vectors, but that
only proves RustyBGP agrees with itself and with one other
implementation's encoder in isolation. CZ.NIC ships no official BIRD
container image, so building one from source is the only way to bring
a third, independent OLD BGP speaker into the existing Docker-Compose
e2e harness.

The added scenario chains gobgp-new -> rusty -> bird-old -> frr-new,
with bird-old's session toward rusty running `enable as4 off`. This
exercises both directions with a live peer instead of canned bytes:
rusty downgrading a wide-AS route for bird-old (encode), and bird-old
downgrading a different wide-AS route for rusty (decode) -- the latter
validates RustyBGP's reconcile logic against AS4_PATH bytes it did not
generate itself.

Assisted-by: Claude Sonnet 5 <noreply@anthropic.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>