v-m/vmagent: Scrape public clouds to test Internet

Scraping the public DNS servers doesn't work anymore since the firewall
routes traffic through Mullvad.  Pinging public cloud providers should
give a pretty decent indication of Internet connectivity.  It will also
serve as a benchmark for the local DNS performance, since the names will
have to be resolved.

victoria-metrics/blackbox.yml

@@ -26,6 +26,8 @@ modules:
   icmp:
     prober: icmp
     timeout: 5s
+    icmp:
+      preferred_ip_protocol: ip4
   smtp:
     prober: tcp
     tcp: