docs: fix wrong env var to be set [#1166]

[tinaboyce]

• Fixes issue incorrect environment variable in docs: SAML_DUO_MFA_OPTION should be SAML2AWS_DUO_MFA_OPTION #1166 by updating the docs to reference the correct env var
• Add references to "SAML2AWS_DUO_MFA_OPTION" env var in the help command flag
• Update README with latest help command text
• Change references to previously named Mac OSes to current OS name
• Add GoReleaser requirement to macOS building requirements

Issue: [#1166]

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (f183682) 39.26% compared to head (e252562) 39.26%.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1171   +/-   ##
=======================================
  Coverage   39.26%   39.26%           
=======================================
  Files          53       53           
  Lines        8030     8030           
=======================================
  Hits         3153     3153           
  Misses       4454     4454           
  Partials      423      423           

Flag	Coverage Δ
unittests	39.26% <ø> (ø)

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