refactor: remove dependence on env vars and rely on forge injected entities

[ramenforbreakfast]

Foundry has since been updated to the point where relying on environment variables to inject accounts is no longer necessary.

BatchScript.sol is a forge script and thus already has a signer and chainId configured when the script is run. There should be no need to rely on environment variables to duplicate what is already natively injected by forge.

In addition, vm.sign has been recently updated to allow for signing of messages directly via whatever wallet has been selected via forge script options, thus there is no need for complex string logic to assemble an external cast call anymore. forge already allows usage of plaintext keys, keystores, and hardware wallets so this shouldn't result in any loss of functionality.

Haven't tested it with hardware wallets though, not sure if vm.sign can correctly identify a --sender with a ledger/trezor provided via options.