vim: Fix editing crontabs with crontab -e

vim: Fix editing crontabs with crontab -e

Per default, vim creates a new file and keeps the original file as backup.
This leads to crontab checking the backup file for changes instead of the
one actually being edited, resulting in any changes being silently dropped.
Since busybox's crontab saves temporary crontab files in "/etc/crontabs.*"
vim's default of "backupskip=/tmp/*" doesn't circumvent this.

This commit fixes this issue by appending "/etc/crontabs.*" to "backupskip".

Signed-off-by: Florian Fieber <florian@florianfieber.de>