Add testnet flags to combine command

[xenowits]

🎯 Problem to be solved

The current charon combine command doesn't support testnet flags and errors like the below:

charon-distributed-validator-cluster % docker run -u $(id -u):$(id -g) --rm -v "$(pwd)/:/opt/charon" obolnetwork/charon:v0.19.1-rc1 combine    
12:17:55.905 INFO cmd        Recombining private key shares           {"input_dir": "/opt/charon/.charon/cluster", "output_dir": "/opt/charon/validator_keys"}
12:17:55.918 ERRO cmd        Fatal error: cannot open manifest file: manifest load error: load dag from disk: couldn't load cluster from legacy lock file: cluster lock signature verification failed. Run with --no-verify to bypass verification at own risk: invalid definition: invalid fork version {"name": "node0", "fork_version": "0x10000038"}
        eth2util/network.go:110 .networkFromForkVersion
        eth2util/network.go:115 .ForkVersionToChainID
        cluster/eip712sigs.go:74 .digestEIP712
        cluster/definition.go:182 .VerifySignatures
        cluster/lock.go:145 .VerifySignatures
        cmd/combine/combine.go:268 .verifyLock
        cmd/combine/combine.go:230 .func1
        cluster/manifest/load.go:103 .loadDAGFromLegacyLock
        cluster/manifest/load.go:47 .LoadDAG
        cluster/manifest/load.go:26 .LoadCluster
        cmd/combine/combine.go:229 .loadManifest
        cmd/combine/combine.go:61 .Combine
        cmd/combine.go:51 .newCombineFunc
        cmd/combine.go:28 .func1
        cmd/cmd.go:80 .func1
        main.go:19 .main

🛠️ Proposed solution

Add testnet flags to the charon combine command.

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