`target/cxxbridge/` not created when `CARGO_TARGET_DIR` is set

[pantos9000]

Hi,

I was able to reproduce this with the demo in the latest version, as well as the current main branch.

• cd into the demo folder
• cargo build works (../target/cxxbridge/ exists)
• cargo build --target-dir foo works (./foo/cxxbridge/ exists)
• CARGO_TARGET_DIR=bar cargo build does not work (./bar/ exists, but ./bar/cxxbridge/ does not)

This is easy to work around with the --target-dir parameter, but the env var is nicer for creating an environment where all cargo commands use the same target dir. The parameter is easy to forget.

BTW, awesome crate! Thanks for creating it!