fix: toolkit: make email regex pattern match pve-common

fix: toolkit: make email regex pattern match pve-common

`proxmoxMail` used its own regex pattern to validate emails. that
meant certain email addresses were rejected by the front-end that
were accepted by the backend that uses the functionality from
`pve-common`. examples include the following:

- "user@host.test-tld"
- "user-@host.testtld"
- "user@host"

reported on the forum: https://forum.proxmox.com/threads/gui-bugulance-using-the-user-add-gui-interface.114743/

Signed-off-by: Stefan Sterz <s.sterz@proxmox.com>