Updated approach to include premined contracts

Add support for large preloads and premines (#153)

We're using the Kurtosis Ethereum package to deploy chains in testing.
Some of these chains have large numbers of preloaded contracts and
premine addresses. Since the preloads and premines are loaded into the
entrypoint via env vars, they can sometimes overflow the maximum arg
space available to Bash and cause 'Argument list too long' errors. This
PR removes the preloads/premines from the env file so that the script
can proceed as originally intend, and implements a custom envsubst
script in Python for use with execution layer genesis generation.