Fix race condition in email output generation

[Shotte]

Summary

Fixes a race condition in the email output generation of check-updates.sh.

The update check output is written through asynchronous tee process substitutions. The EXIT trap may inspect and send mail-output before these processes have finished writing, resulting in empty or incomplete email notifications.

Changes

• Store the complete check result in check-output through one tee process.
• Preserve and restore the original stdout.
• Wait for the output process to finish before generating the email.
• Build mail-output synchronously from the completed check-output.
• Send the no-updates notification to the configured EMAIL_USER.

Validation

Tested on:

• Proxmox VE 9.2
• pve-manager 9.2.5
• Ultimate Updater Check 1.7.9

Before the change:

check-output: 196 bytes
mail-output: 0 bytes

After the change:

check-output: 196 bytes
mail-output: 140 bytes

The generated email contained the complete update summary after the change.

Validation performed:

bash -n check-updates.sh

The syntax check completed successfully.

Fixes #289