streamline the Send-MailKitMessage installation

install.ps1

@@ -60,5 +60,8 @@ else {
     Write-Warning "[[Scheduler]] Backup task not scheduled: there is already a task with the name '$backup_task_name'."
 }
 
-# Install Send-MailKitMessage module
-Install-Module Send-MailKitMessage -Repository PSGallery -Scope AllUsers
+# Install NuGet and Send-MailKitMessage module (by force)
+if ($PSVersionTable.PSVersion.Major -eq 5) {
+    Install-PackageProvider -Name NuGet -Force
+}
+Install-Module Send-MailKitMessage -Repository PSGallery -Scope AllUsers -Force