Clippy leaves cargo running in the background on SIGTERM

[Nemo157]

When you kill cargo-clippy via SIGTERM it will leave a detached cargo check process running in the background. If, for example, the clippy instance was launched by your editor and killed when you exited it this can result in a long duration of Blocking waiting for file lock on build directory when you attempt to run cargo test.

Test reproduction:

• Run cargo clean && cargo clippy on a project that will take a decent while to check
• In another terminal run killall cargo-clippy to send SIGTERM
• In the original terminal watch the process exit, but the output from checking continue to be printed

This appears to be already fixed by #3837, so maybe not worth trying to fix the current subcommand, I'm mostly opening this as informational in case others run into it as well.