contracts-bedrock: update `.envrc.example`

We need to update the superchain getting started guide
as well as the deploy script so that the guide shows usage
of the deploy script where it only deploys a set of proxies
and then initializes them to the standard set of implementations.
For now, use `openssl` to create a random string to use as
the `create2` salt so that individual deployments do not
clash and fail. A problem was happening where contract deployments
were failing due to contract addresses colliding.

packages/contracts-bedrock/.envrc.example

@@ -10,3 +10,9 @@ export DEPLOYMENT_CONTEXT=getting-started
 # Optional Tenderly details for a simulation link during deployment
 export TENDERLY_PROJECT=
 export TENDERLY_USERNAME=
+
+export ETHERSCAN_API_KEY=
+
+# Optional create2 salt for deterministic deployment of
+# contract implementations
+export IMPL_SALT=$(openssl rand -hex 32)