Linea: set sequencer URLs (rpc.linea.build / rpc.sepolia.linea.build)

maru v1.2.x strictly validates l2-eth-api-endpoint as a URL and crashes when SEQUENCER
env var is empty. Old maru tolerated empty. Fixes 4 stuck Linea instances on
de-27, de-32, us-40 — all besu/geth running but maru crash-looping with config error
'Required type class java.net.URL could not be decoded from a String value: '

linea/geth/linea-sepolia-geth-pruned-pebble-path.yml

@@ -123,7 +123,7 @@ services:
       - IP=${IP}
       - L1_RPC=${ETHEREUM_SEPOLIA_EXECUTION_RPC}
       - P2P_PORT=19190
-      - SEQUENCER=
+      - SEQUENCER=https://rpc.sepolia.linea.build
     entrypoint: [/bin/bash, -c]
     command:
       - |