rustpkg creates temporary directories but does not delete them

[ben0x539]

rustpkg calls extra::tempfile::mkdtemp (mostly in rustpkg/tests but also in rustpkg::{package_source,version}) and doesn't ever remove the resulting directories.

/tmp grows by 8MiB every time I run make check, we should fix it before I run out of disk space and have to reboot! ;)

[ben0x539]

I was looking into fixing this myself (eg. like this) but I decided I want to see if there's interest in changing mkdtemp to return an RAII wrapper so I don't have to do the RAII in rustpkg code (#9763). But I also don't want to forget about this!