generate_report didn't work

[rhcu]

https://github.com/mozilla/coverage-crawler/blob/31c8b1841a931e3a4d15013f8463be9475b34f13/coverage_crawler/latest_cov_build.py#L107 - I wanted to run a crawler from the scratch and downloaded artifacts. However, this line resulted in tests_report.json weighing 157 bytes. Not sure whether this is relevant to grcov, or here.

[marco-c]

Maybe there's a regression from one of the latest changes in this repository?
Could you try to pinpoint where the problem is coming from?

[marco-c]

For example, by adding some logging to the script to see how grcov is executed (printing cmd at https://github.com/marco-c/firefox-code-coverage/blob/master/firefox_code_coverage/codecoverage.py#L200).