`update-tests-metadata` is flaky

Job #307895203 failed for 40cfd00d:

scripts/merge-reports:16:in `read': No such file or directory @ rb_sysopen - knapsack/rspec*_pg9_*.json (Errno::ENOENT)
	from scripts/merge-reports:16:in `block in <main>'
	from scripts/merge-reports:15:in `each'
	from scripts/merge-reports:15:in `<main>'
Loading knapsack/report-master.json...

I see downloading artifacts lines above and when I manually browse the pg9 job artifacts I also see that there is a report there. Not sure why this is happening.

Other instance of failure:

https://gitlab.com/gitlab-org/gitlab/-/jobs/308033750

cc @rymai @godfat