Codecov: use go-acc for better code coverage on `master`
Created by: unknwon
go-acc ./… && go tool cover -func coverage.txt | grep total
Although it gives more accurate code coverage of our tests, it takes significantly longer (>12m) than simple go test -race ./… (~2m). So instead of enabling it on every branch, we can only enable it on master which represents the source of truth for the code coverage.