fix: check allowed options for flags provided through env

[peterjah]

This enable the options test for flags provided through environment variables

[salesforce-cla]

Thanks for the contribution! Before we can merge this, we need @peterjah to sign the Salesforce.com Contributor License Agreement.

[codecov]

Codecov Report

Merging #64 into master will decrease coverage by 7.46%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master      #64      +/-   ##
==========================================
- Coverage   91.35%   83.89%   -7.47%     
==========================================
  Files          11       11              
  Lines         324      329       +5     
  Branches       85       90       +5     
==========================================
- Hits          296      276      -20     
- Misses         12       37      +25     
  Partials       16       16

Impacted Files	Coverage Δ
src/parse.ts	90.14% <0%> (-1.29%)	⬇️
src/validate.ts	31.42% <0%> (-65.45%)	⬇️

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