[CI] Increase stream buffer limit for build subprocess

Summary: This fixes the error currently seen in secp256k1 tests: https://build.bitcoinabc.org/viewLog.html?buildId=88041&buildTypeId=BitcoinABC_Master_BitcoinAbcMasterSecp256k1&tab=buildLog&_focus=256

Test Plan: Test secp256k1 in CI with this patch: https://build.bitcoinabc.org/viewLog.html?buildId=88107&buildTypeId=BitcoinABC_BitcoinAbcStaging&tab=buildResultsDiv

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6753