Add support for CYPHAL_PATH

[pavel-kirienko]

It seems like Nunavut expects the env vars to contain the list of root namespace directories, not the list of directories that contain root namespace directories in them. Observe:

outer/
    scotec/             <-- a DSDL root namespace
        Thing.1.0.dsdl
    ns/                 <-- a DSDL root namespace
        Foo.1.0.dsdl

CYPHAL_PATH would contain outer while your path variables would contain outer/scotec:outer/ns.

Originally posted by @pavel-kirienko in #303 (comment)