Add another HTTPS probe for bsc.es

As all other HTTPS probes pass through the opsproxy01.bsc.es proxy, we
cannot detect a problem in our proxy or in the BSC one. Adding another
target like bsc.es that doesn't use the ops proxy allows us to discern
where the problem lies.

Instead of monitoring https://www.bsc.es/ directly, which will trigger
the whole Drupal server and take a whole second, we just fetch robots.txt
so the overhead on the server is minimal (and returns in less than 10 ms).

Reviewed-by: Aleix Roca Nonell <aleix.rocanonell@bsc.es>

m/hut/monitoring.nix

@@ -105,6 +105,7 @@
         params = { module = [ "http_2xx" ]; };
         static_configs = [{
           targets = [
+            "https://www.bsc.es/robots.txt"
             "https://pm.bsc.es/"
             "https://pm.bsc.es/gitlab/"
             "https://jungle.bsc.es/"