Compatibility combo: ZkSync support

[brickpop]

Allowing to override just variables from the root project

Adding support for ZkSync

• just test, just deploy, just run, and all related commands now automatically use forge-zksync when the active network is zksync or zksync-sepolia — no separate -zksync recipe variants needed
• New hidden recipe just setup-zksync installs foundry-zksync alongside standard Foundry, naming the binaries forge-zksync / cast-zksync so both coexist
• New private recipe resolve-forge selects the right binary from the chain ID (hardcoded logic — not from env, no injection risk)
• resolve-build-params simplified: only emits compiler flags (--zksync); binary selection moved to resolve-forge
• Both resolve-forge and resolve-build-params are now self-contained (load their own network env)
• just setup now correctly runs foundryup -v stable after the installer, with an explicit PATH fix so it works in a fresh shell

Breaking changes

• DEPLOYMENT_PRIVATE_KEY renamed to DEPLOYER_KEY

Other:

• New Environment variables section in README listing all variables exposed by network configs and the secrets expected from the user
• just setup-zksync documented in the Debug helpers section