Fix reading fallback values for providers, set environment variables

[jhrozek]

Summary

This PR fixes reading the fallback values for providers to read the keys one-by-one
to allow setting environment variables - I was trying to be too smart and check the
presence of the fallback key by checking viper.IsSet(provider) and then unmarshalling
viper.UnmarshallKey(provider) but that doesn't work well with environment variables.

Also includes PR #3427 just adjusts the variable names.

Change Type

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• Bug fix (resolves an issue without affecting existing features)
• Feature (adds new functionality without breaking changes)
• Breaking change (may impact existing functionalities or require documentation updates)
• Documentation (updates or additions to documentation)
• Refactoring or test improvements (no bug fixes or new functionality)

Testing

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Attach screenshots if helpful.

Review Checklist:

• Reviewed my own code for quality and clarity.
• Added comments to complex or tricky code sections.
• Updated any affected documentation.
• Included tests that validate the fix or feature.
• Checked that related changes are merged.

[coveralls]

coverage: 52.03% (+0.05%) from 51.98%
when pulling 3963ac4 on jhrozek:revert
into ee7bec4 on stacklok:main.