Fix asset loading in debug launch config on Linux

[caspark]

Problem

When running the Debug launch configuration in vscode on Linux, the assets are not loaded, so the menu button is not visible. The Quick Launch launch configuration works fine.

Diagnosis

1. Bevy logs show lines like 2023-08-10T11:39:58.234289Z WARN bevy_asset::asset_server: encountered an error while reading an asset: path not found: /home/foo/src/bevy_game/target/debug/assets/fonts/FiraSans-Bold.ttf , indicating that Bevy isn't detecting the root folder correctly.
2. The Debug launch configuration is setting CARGO_MANIFEST_DIR as (one of the variables that) Bevy expects to see, however cat "/proc/$(pgrep bevy_game)/environ" | tr '\0' '\n' | grep CARGO returns no results, indicating that the environment variable isn't actually getting set.
3. Vscode docs say Properties defined in an operating system specific scope override properties defined in the global scope. so the .linux.env setting is overriding the .env setting (rather than being merged with it, as one might reasonably expect).

Fix

Duplicate CARGO_MANIFEST_DIR into .linux.env.

Other notes

• any particular reason you picked CARGO_MANIFEST_DIR over BEVY_ASSET_ROOT?
• the Debug unit tests config sets LD_LIBRARY_PATH to the same value for all platforms rather than having a Linux specific override, which on the one hand seems like a mistake (the value is decidedly Linux-specific), but on the other hand is harmless (the envvar has no meaning on non-Linux platforms) - so a reasonable alternative fix is to just do the same thing for the Debug launch config.

[NiklasEi]

I do not use vscode. The setup was contributed in #9.

@TheRawMeatball any opinion on this?