cmd, common, eth: copy constants after get chain id

[gzliudan]

Proposed changes

The PR #850 sets network ID automatically. And the PR #846 copy constants during XDC startup. But when the flag NetworkIdFlag is not provide by script, the function common.CopyConstans in SetEthConfig does not work. So I move it into the function New in the file eth/backend.go after networkID has been determined.

Types of changes

What types of changes does your code introduce to XDC network?
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• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)
• Regular KTLO or any of the maintaince work. e.g code style
• CICD Improvement

Impacted Components

Which part of the codebase this PR will touch base on,

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• Consensus
• Account
• Network
• Geth
• Smart Contract
• External components
• Not sure (Please specify below)

Checklist

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• This PR has sufficient test coverage (unit/integration test) OR I have provided reason in the PR description for not having test coverage
• Provide an end-to-end test plan in the PR description on how to manually test it on the devnet/testnet.
• Tested the backwards compatibility.
• Tested with XDC nodes running this version co-exist with those running the previous version.
• Relevant documentation has been updated as part of this PR
• N/A