bgpd, tests: don't send local nexthop from rr client

bgpd, tests: don't send local nexthop from rr client

AS 65000  | AS 65001
          |
      RR  |
       |  |
R1 --- | --- R2
          |

When r1 peer is an iBGP route reflector client of rr and r2 peer is a
eBGP neighbor of rr, and all three routers shares the same network, r2
receives announcements coming from r1 with a IPv6 link-local nexthop
from rr. This is incorrect as r2 should send traffic to r1 without
involving rr.

Do not send an IPv6 link-local nexthop if the originating peer is a
route-reflector client.

Link: https://github.com/FRRouting/frr/pull/16219#issuecomment-2397425505
Link: https://github.com/FRRouting/frr/pull/17037#discussion_r1792529683
Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>