dev: Use github.com/caitlinelfring/go-env-default package

[caitlinelfring]

Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
Switch from util package for accessing environment variables to a library: https://github.com/caitlinelfring/go-env-default

What is the current behavior? (You can also link to an open issue here)

What is the new behavior (if this is a feature change)?

Does this PR introduce a breaking change? (What changes might users need to make due to this PR?)

Other information:

[codecov]

Codecov Report

Merging #87 (d0fc21f) into main (e6c5bed) will decrease coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #87      +/-   ##
==========================================
- Coverage   96.21%   96.16%   -0.06%     
==========================================
  Files          21       20       -1     
  Lines         582      573       -9     
==========================================
- Hits          560      551       -9     
  Misses         13       13              
  Partials        9        9              

Impacted Files	Coverage Δ
pkg/parser/parser.go	100.00% <100.00%> (ø)
pkg/printer/printer.go	100.00% <100.00%> (ø)

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