Change arbitrum-sepolia-testnet to either arbitrum-testnet or arbitrum-sepolia

[armsves]

When using the cli and having to choose arbitrum sepolia, the chain identification is a bit long, it would be better devex to use either arbitrum-testnet or arbitrum-sepolia

just a small change in line 91 of file:
https://github.com/iExecBlockchainComputing/iapp/tree/main/cli/src/config/config.ts

Thanks!