Fix X-Robots-Tag not working properly

HAproxy 1.6 requires setting the Host header via a temporary variable for the backend to use it: https://stackoverflow.com/a/35417254

Otherwise, we see this error message:

Feb 19 21:46:47 fe-01-lb-gstg haproxy-systemd-wrapper[22099]: [WARNING] 049/214647 (30359) : parsing [/etc/haproxy/haproxy.cfg:317] : acl 'is_canary_host' will never match because it only involves keywords that are incompatible with 'backend http-response header rule'