grafana: Set the Host header to account for CSRF changes[1] in v8.3.5

Without this fix all the Prometheus dashboards failed loading data with:
"Error updating options: origin not allowed".

[1] https://github.com/grafana/grafana/issues/45117#issuecomment-1033842787

roles/grafana/templates/nginx.d.conf.j2

@@ -41,6 +41,7 @@ server {
 
 {% set proxy -%}
         proxy_pass http://grafana;
+        proxy_set_header Host $host;
         proxy_set_header X-Forwarded-For      $remote_addr;
         proxy_set_header Upgrade $http_upgrade;
         proxy_set_header Connection $connection_upgrade;