Ignores CARGO_TARGET_DIR when downloading electron

[marcianx]

When running cargo node --target electron ..., cargo-node downloads electron into target/electron within the crate directory instead of within CARGO_TARGET_DIR. When running, it seems to fail to find the downloaded electron directory and results in a build failure with "No such file or directory".

Unsetting CARGO_TARGET_DIR resolves the issue for the electron build.

Nice tool, by the way!