Invalid JWT signature if --el is not specified

[lgaroche]

Bug description
By default, the nimbus node connects to a local EL client on http://127.0.0.1:8551, if the --el option is omitted. When doing so, the JWT is rejected by the EL (I'm using Reth).
If I explicitly specify the command line option --el=http://127.0.0.1:8551, then the clients can communicate well.

To Reproduce
Nimbus version: v23.11.0-3a527d-stateofus
Reth version: 0.1.0-alpha.13
OS: Rocky Linux 9 5.14.0-362.8.1.el9_3.x86_64
Running EL and CL on the same host. IPTables are open (all ACCEPT)

Reth is running, listening on localhost port 8551.

Start nimbus without --elflag:

/build/nimbus_beacon_node --network:mainnet --jwt-secret=/root/.local/share/reth/mainnet/jwt.hex

Bug: JWT error:
In Nimbus logs: WRN 2023-12-12 16:07:26.078+01:00 Failed to exchange transition configuration topics="elmon" url=http://127.0.0.1:8551 err=Unauthorized
In Reth logs: 2023-12-12T15:07:26.078366Z ERROR engine::jwt-validator: Invalid JWT: provided signature is invalid

Start nimbus with the flag:

./build/nimbus_beacon_node --network:mainnet --el=http://127.0.0.1:8551 --jwt-secret=/root/.local/share/reth/mainnet/jwt.hex

Then there's no error.

Expected behavior
As the default EL Url is the same as the one I specify, it should work without the flag.

[tersec]

#5671