LD_LIBRARY_PATH init issue at startup

[Lypsolon]

Error Description.
The plugin will have an error when starting the Ayon_Usd plugin with, e.g. Houdini and not having the LD_LIBRARY_PATH env variable set.

Assumed Reason:
Python os module can't be appended to an env variable if this env variable is not set yet.

Possible solution:
Could you check if the variable exists? If it's not set it. If it's set, append to it.
This will cause a check against the current set of env variables. This could be slow, but this could be acceptable as it runs before the software startup and only once.

[cuID:[AY-5681](https://app.clickup.com/t/86bz1eq9f]

[antirotor]

should be fixed by #5