Change additional cert file name to avoid conflicts

if additional cert bundle does not include all public certs custom CA's will work but public CAs will not when file is named ca-certificates.crt.  
It appears that kaniko is using / creating that file during execution to include built in public certs. 

doc/ci/docker/using_kaniko.md

@@ -90,7 +90,7 @@ store:
     - |
       echo "-----BEGIN CERTIFICATE-----
       ...
-      -----END CERTIFICATE-----" >> /kaniko/ssl/certs/ca-certificates.crt
+      -----END CERTIFICATE-----" >> /kaniko/ssl/certs/additional-ca-cert-bundle.crt
 ```
 
 ## Video walkthrough of a working example