[CI] Increase build-coverage timeout

Summary:
The current 1h timeout is OKish but has almost no margin, so this diff
adds 20 minutes more.

Test Plan: Run the build-coverage build plan.

Reviewers: #bitcoin_abc, majcosta, jasonbcox

Reviewed By: #bitcoin_abc, majcosta, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7116