contracts-bedrock: simplify hh config for mainnet

hardhat barfs if we pass the zero key in, not sure why we
do it everywhere. default to using the 'test test ...' key
at index 0 if the private key is not passed instead.
this is useful for mainnet migration rehearsals and since
the mainnet contracts have already been deployed, unlikely
this will be used again and if used again the private key
env var MUST be passed anyways

packages/contracts-bedrock/hardhat.config.ts

@@ -33,7 +33,10 @@ const config: HardhatUserConfig = {
     // NOTE: The 'mainnet' network is currently being used for mainnet rehearsals.
     mainnet: {
       url: process.env.L1_RPC || 'https://mainnet-l1-rehearsal.optimism.io',
-      accounts: [process.env.PRIVATE_KEY_DEPLOYER || ethers.constants.HashZero],
+      accounts: [
+        process.env.PRIVATE_KEY_DEPLOYER ||
+        'ac0974bec39a17e36ba4a6b4d238ff944bacb478cbed5efcae784d7bf4f2ff80'
+      ],
     },
     devnetL1: {
       live: false,