Provide a default value for the BEVY_ASSET_PATH

[dbidwell94]

I'm trying to setup a build process using cross-rs. Unfortunately, I have to manually specificy the BEVY_ASSET_PATH before the command. Also unfortunately, it's ran in a docker container so an absolute path is unhelpful. If I pass in BEVY_ASSET_PATH=./assets then this mod tries to load assets from its example dir (which doesn't exist). You can reproduce this now by running BEVY_ASSET_PATH=./assets cargo run and seeing the error outputs from bevy_embedded_assets.

I propose either 2 solutions.

1 - make the env var of BEVY_ASSET_PATH relative to the cargo workspace root

2 - default the value of BEVY_ASSET_PATH to be the root of the project with /assets (as that's the bevy default)