`ZENOH_CONFIG` is only used in `Config::from_env`

[fuzzypixelz]

The API around DEFAULT_CONFIG_PATH_ENV doesn't make much sense. We have:

• https://docs.rs/zenoh/latest/zenoh/struct.Config.html#associatedconstant.DEFAULT_CONFIG_PATH_ENV
• https://docs.rs/zenoh/latest/zenoh/struct.Config.html#method.from_env

Thus DEFAULT_CONFIG_PATH_ENV only exists to document what environment variable Config::from_env will use to lookup a file.

This led to some confusion (see "Loading Zenoh plugins" on the community support channel) where users expect this to also override the configuration file used by a session.

We should consider extending the use of ZENOH_CONFIG to fit this function.

System info

• ref: 1.6.2