Extract mapping lookup from loop

[arisudesu]

Some repeated calls I've found while looking through prepare_email logic.

[codecov-commenter]

Codecov Report

Merging #566 (ff52e69) into master (9a87d73) will increase coverage by 0.00%. The diff coverage is 41.17%.

@@           Coverage Diff           @@
##           master     #566   +/-   ##
=======================================
  Coverage   52.89%   52.90%           
=======================================
  Files         126      126           
  Lines       12975    12977    +2     
=======================================
+ Hits         6863     6865    +2     
  Misses       5468     5468           
  Partials      644      644           

Flag	Coverage Δ
integration	34.04% <41.17%> (+0.04%)	:arrow_up:
unit	54.42% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
internal/authz/lookup.go	48.14% <41.17%> (+4.14%)	:arrow_up:

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[foxcpp]

Thanks!