Add EL8 to metadata.json

[trevor-vaughan]

No description provided.

[binford2k]

confirmed out of band that the committer has validated on these platforms.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@3cf2bef). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #106   +/-   ##
=========================================
  Coverage          ?   83.53%           
=========================================
  Files             ?        3           
  Lines             ?      164           
  Branches          ?        0           
=========================================
  Hits              ?      137           
  Misses            ?       27           
  Partials          ?        0

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