Previously, the env endpoint would use the entire environment to
get the value of each property in a source. This meant that when
there were multiple sources with the same property, the value from
the source with the highest precedence would be used for every
source that contains the property.

This commit update the endpoint to retrieve the value from the
property source that is being described, rather than resolving it
against all the environment's property sources.

Closes gh-10883