fix(config): allow using environment variables without config file present

[orhun]

Motivation and Context

fixes #777

How Has This Been Tested?

Locally.

Types of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (no code change)
• Refactor (refactoring production code)
• Other

Checklist:

• My code follows the code style of this project.
• I have updated the documentation accordingly.
• I have formatted the code with rustfmt.
• I checked the lints with clippy.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 36.97%. Comparing base (4b33e7e) to head (d4b3b85).

Files	Patch %	Lines
git-cliff-core/src/embed.rs	0.00%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #783      +/-   ##
==========================================
- Coverage   37.03%   36.97%   -0.06%     
==========================================
  Files          20       20              
  Lines        1542     1542              
==========================================
- Hits          571      570       -1     
- Misses        971      972       +1     

Flag	Coverage Δ
unit-tests	36.97% <0.00%> (-0.06%)	⬇️

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