rust.vim version: master (889b9a7515db477f4cb6808bef1769e53493c578)

Steps to reproduce:

Trigger :make from Vim while some other build of the same project is in progress. It happens all the time if one uses rust-analyzer, saves a Rust source file and quickly triggers make, like :w | :make which I do often: once the file is saved, rust-analyzer performs the build automatically, and by the time Vim tries to build it as well, it has to wait for the other build to finish, and cargo prints lines like Blocking waiting for file lock on package cache.

Expected vs. actual behavior:

Expected those lines to be ignored by Vim, and not included in quickfix, however they are included in quickfix, and even if the build succeeded, the quickfix is non-empty, making it look like there are issues:

||     Blocking waiting for file lock on package cache
||     Blocking waiting for file lock on package cache
||     Blocking waiting for file lock on build directory