Fix abcore mainnet: wire client_version to build arg and use equals form for --config flag

- Fix ABCORE_MAINNET_GETH_VERSION default from empty to v1.13.15-abcore-1.2
- Fix command flag from '--config /config/config.toml' to '--config=/config/config.toml'

Both issues caused first-deploy failures on rpc-de-32:
1. Empty version default caused curl exit 22 on binary download
2. Space-separated flag/value caused geth crash-loop with 'flag provided but not defined: -config'

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

abcore/geth/abcore-mainnet-geth-pruned-pebble-path.yml

@@ -37,7 +37,7 @@ services:
       context: ./abcore
       dockerfile: abcore.Dockerfile
       args:
-        ABCORE_MAINNET_GETH_VERSION: ${ABCORE_MAINNET_GETH_VERSION:-}
+        ABCORE_MAINNET_GETH_VERSION: ${ABCORE_MAINNET_GETH_VERSION:-v1.13.15-abcore-1.2}
     sysctls:
       # TCP Performance
       net.ipv4.tcp_slow_start_after_idle: 0       # Disable slow start after idle
@@ -62,7 +62,7 @@ services:
       - CHAIN_NAME=mainnet
     entrypoint: [/usr/local/bin/init.sh]
     command:
-      - --config /config/config.toml
+      - --config=/config/config.toml
       - --datadir=/root/.ethereum
       - --db.engine=pebble
       - --gcmode=full