matrix: add el8 tag and make it test-install

[trws]

problem: we're not hooking up to the el8 container from core properly

solution: tag the container after the test, and match the core container with test-install

This is meant to match with #flux-core/6401 and ensure we propagate the el8 container all the way through.

[milroy]

Looks good; thanks for the quick PR.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.3%. Comparing base (996f999) to head (ba57560).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #1311    +/-   ##
=======================================
  Coverage    75.3%   75.3%            
=======================================
  Files         111     111            
  Lines       15300   15979   +679     
=======================================
+ Hits        11531   12048   +517     
- Misses       3769    3931   +162     

see 70 files with indirect coverage changes