Update omnibus builds to Ruby 2.6.6

[tas50]

2.6.5 has 2 CVEs in it:

CVE-2020-10663: Unsafe Object Creation Vulnerability in JSON (Additional fix)
CVE-2020-10933: Heap exposure vulnerability in the socket library

Signed-off-by: Tim Smith tsmith@chef.io

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[chef-expeditor]

Hello tas50! Thanks for the pull request!

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   c. Cases exist where a PR is neither aligned to Chef InSpec's product roadmap, or something the team can own or maintain long-term. In these cases, the maintainer will provide justification and close out the PR.

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