2924: Modify ordering of drops in check watcher to only ever have one cargo r=matklad a=kiljacken
Due to the way drops are ordered when assigning to a mutable variable we
were launching a new cargo sub-process before letting the old one quite.
By explicitly replacing the original watcher with a dummy first, we
ensure it is dropped and the process is completed, before we start the
new process.
Co-authored-by: Emil Lauridsen <mine809@gmail.com>
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