Promote installer config options as env vars explicitly

[acelaya]

#2201 showed that the fact that loading the config also promotes installer options as env vars is something easy to forget.

This PR changes that so that promoting config options is done explicitly, in case any of them needs to be read anywhere outside of config files, reducing the changes of reading the env var too early.

Closes #2208

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.73%. Comparing base (447ccca) to head (15b53ef).
Report is 5 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff             @@
##             develop    #2211   +/-   ##
==========================================
  Coverage      94.73%   94.73%           
  Complexity      1551     1551           
==========================================
  Files            269      269           
  Lines           5200     5200           
==========================================
  Hits            4926     4926           
  Misses           274      274           

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