Use environment variables when error is strict

[lahiguera]

Summary

Fix minor bug in readConfig function when checking the error of unmarshalSemiStrictly.

Motivation

This will allow us to use a configuration that has an unmarshalling strict error, i.e. when the configuration have unknown fields, and the validate-config and validate-config-strict command-line flags are both false.

Test plan

Automated testing.

Rollout/monitoring/revert plan

None

[stripe-ci]

Gerald Rule: Copy Observability on Veneur, Unilog, Falconer pull requests

cc @stripe/observability @stripe/observability-stripe

[asf-stripe]

Handing this over to aditya! (:

[CLAassistant]

All committers have signed the CLA.