[refactor] set the -ecash CLI argument earlier in startup

Summary: init.cpp isn't executed for bitcoin-tx, for example, which also needs the -ecash switch available

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9655