let rez-context detect sourced context

Sometimes rez is used to configure the current env, rather than a subshell. That can be done like so:

source <(rez-env -o - | rez-context -i -);

Currently rez-context will fail in this env. Instead, it should still show what info it can (and there's still a lot that can be retrieved from the REZ_* envvars).

Note that this will have to be enabled with a new flag (--from-env or similar), since existing bash code could be using return code from rez-context to detect if a context is active, and we don't want to break backwards compatibility.

Start with the rez-context command and reproduce the failure using the sourced command shown in the issue. Trace how it handles the REZ_* environment variables and define the new --from-env behavior. Done means it reports available context information from the environment while preserving existing return-code behavior without the flag.

Written by the indexing model from the issue text.