fix: allow env var to override non-zero struct values (#364)

[jyr94]

Fixes #364

What this does

• Ensures that environment variables override struct values,
  even when SetDefaultsForZeroValuesOnly is true.
• Adjusts get(...) to return a flag indicating the value source.
• Modifies setField to prioritize env vars.
• Adds a test that reproduces the bug.

Why

The current logic prevents environment values from being applied
when the struct field is non-zero, which is not the intended behavior.

[kukymbr]

@caarlos0 hi!
No pressure, but we really need this 🥹

[ZhuJincheng-git]

I think we can just directly check isDefault here.