Revert "Use `/tmp` as the working directory"

This reverts commit c6c8488e. This is necessary since setting WorkingDir=/tmp seems to mount a new tmpfs over /tmp, which "erases" some files that are placed there during molecule testing. Moreover, this setting is only used to ensure that core dump files end up in a location that will be cleaned up regularly.

The coredump handler in the dependent roles will be modified to specify the path /var/tmp instead, and not the current working directory.