feat: add network input for multi-chain builds

[hudsonhrh]

Summary

• Adds an optional network input to the reusable subgraph CI/CD workflow
• When provided, passes --network <name> to both graph build and graph deploy commands
• Enables deploying the same subgraph codebase to multiple chains (e.g., sepolia + base-sepolia) as separate Graph Studio subgraphs

Usage

jobs:
  subgraph-sepolia:
    uses: BreadchainCoop/subgraphform/.github/workflows/_subgraph-cicd.yml@main
    with:
      subgraph-name: poa-sepolia
      working-directory: pop-subgraph
      network: sepolia

  subgraph-base-sepolia:
    uses: BreadchainCoop/subgraphform/.github/workflows/_subgraph-cicd.yml@main
    with:
      subgraph-name: poa-base-sepolia
      working-directory: pop-subgraph
      network: base-sepolia
      run-tests: false

Test plan

• Existing callers without network input should work unchanged (defaults to empty string, no flag passed)
• Callers with network: sepolia should pass --network sepolia to build and deploy
• Callers with network: base-sepolia should pass --network base-sepolia to build and deploy

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