Optimize envar trace context access

See: https://github.com/release-engineering/opentelemetry-ext-cli-java/blob/main/src/main/java/com/redhat/resilience/otel/internal/EnvarExtractingPropagator.java#L134

Trace context is pulled from System.getenv() constantly, which is inefficient. It would be better to store that in memory the first time it's read, and simply return that field value for subsequent calls.