Avoid writing derived dosh_host in installers

install.ps1

@@ -475,7 +475,7 @@ escape_key = "^]"
 $hostsConfig = Join-Path $configDir "hosts.toml"
 if ($ForceConfig -or -not (Test-Path $hostsConfig)) {
     $defaultServer = if ($Server) { $Server } else { "user@example.com" }
-    $hostUdp = if ($DoshHost) { $DoshHost } else { $defaultServer }
+    $hostUdpLine = if ($DoshHost) { "dosh_host = `"$DoshHost`"" } else { "# dosh_host = `"server.example.com`"" }
     @"
 # Example:
 # [server]
@@ -487,7 +487,7 @@ if ($ForceConfig -or -not (Test-Path $hostsConfig)) {
 
 [default]
 ssh = "$defaultServer"
-dosh_host = "$hostUdp"
+$hostUdpLine
 port = $Port
 predict = true
 "@ | Set-Content -NoNewline -Encoding utf8 $hostsConfig