Update antivirus.conf to check for 'no' in SKIP_CLAMD

data/conf/rspamd/local.d/antivirus.conf

@@ -1,4 +1,4 @@
-{% if env.SKIP_CLAMD == "n" %}
+{%- if env.SKIP_CLAMD == "n" or env.SKIP_CLAMD == "no" -%}
 clamav {
   # Scan whole message
   scan_mime_parts = false;