[e.cash] Change from Bitgo utxo-lib to ecash-lib on Build page
Summary: Now that it's implemented in Cashtab, we should recommend the new ecash-lib
Test Plan: Run it locally, check link
Reviewers: bytesofman, #bitcoin_abc
Reviewed By: bytesofman, #bitcoin_abc
Differential Revision: https://reviews.bitcoinabc.org/D16100