🐛 fix(config): set_env cross-section substitution precedence

[gaborbernat]

When a set_env section both inherits values via cross-section substitution (e.g., {[testenv]set_env}) and explicitly defines the same variable, the inherited value incorrectly overwrites the explicit one. 🔄 This means configurations like the common pattern of overriding a single variable from a base environment silently fail — the base value always wins.

[testenv]
set_env = OS_TEST_PATH=./tests/unit

[testenv:functional]
set_env =
    {[testenv]set_env}
    OS_TEST_PATH=./tests/functional  # this was silently ignored

The fix ensures that explicitly defined values in a section take precedence over values arriving from cross-section substitution expansion. This matches the intuitive "last definition wins" principle and restores the behavior users expect when layering environment configurations.

⚠️ Users who previously relied on cross-section substitution values overriding explicit values in the same section will see changed behavior. This was almost certainly unintentional in all cases, since placing a value after a substitution reference naturally implies it should override.

Fixes #2831