fix: allow environment overwrites when using built in configurations

[weichweich]

Description

Allow environment variables to overwrite built in configurations.

Motivation and Context

It is not possible to use environment overwrites when using a built in configuration (i.e. git-cliff -c githhub). e.g. setting GIT_CLIFF__CHANGELOG__HEADER wouldn't change the appropriate value of the example configuration.

How Has This Been Tested?

export GIT_CLIFF__CHANGELOG__HEADER="This is a different header"

git-cliff -c github

Screenshots / Logs (if applicable)

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]

Codecov Report

Attention: Patch coverage is 0% with 7 lines in your changes missing coverage. Please review.

Project coverage is 43.19%. Comparing base (dbda091) to head (99902a9).

Files with missing lines	Patch %	Lines
git-cliff-core/src/embed.rs	0.00%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #961      +/-   ##
==========================================
- Coverage   43.35%   43.19%   -0.15%     
==========================================
  Files          21       21              
  Lines        1712     1718       +6     
==========================================
  Hits          742      742              
- Misses        970      976       +6     

Flag	Coverage Δ
unit-tests	43.19% <0.00%> (-0.15%)	⬇️

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