ra leaves empty `proc-macro-srv` dir behind

[matthiaskrgr]

After fuzzing for a couple of days, noticed that inside /tmp/ I had accumulated more than 40.000 directories like proc-macro-srv15c70d-0

rust-analyzer diagnostics . on an empty crate (cargo new) is enough to make the stale folder appear.
I would think that at least in case of the happy path (no ra panic), leftover files are removed before the application finally terminates.

[ChayimFriedman2]

I'm working on a fix (although it won't work on macOS), but as a stopgap you can set RA_DONT_COPY_PROC_MACRO_DLL=1 (that won't work: the files won't be copied but the directory will still be created).