Skip simplecov for vendor/gems/

What does this MR do and why?

Closes Skip Simplecov for vendor/gems in spec/simpleco... (#375652 - closed)

A lot of gems in vendor/gems are forks of existing gems, and their coverage may be less than 100%.

Also we don't report back coverage from the child pipelines, so this creates undercoverage issues

I'm testing this change in !142463 (closed)

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