Option (or default?) to keeping temporary directory if cargo build fails

I've often wanted to troubleshoot a failing build in the generated project. That's made a lot harder by the deletion of the project at the end of the build. It would be nice if the temporary directory could be left in place if it fails.

Start by tracing where the generated project's temporary directory is created and cleaned up around the cargo build failure path. Check how build failures are reported, then define the option or default behavior so the directory remains available for troubleshooting when the build fails.

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