EC2: Fix additional files on small+builder AMIs

The file /usr/local/etc/ssl/cert.pem is not present on "small" and
"builder" AMIs, so we don't need to add it to METALOG.

Fixes: 2b0ffc0ee48c ("EC2: metalog_add missing files from packages")
MFC after: 3 days
Sponsored by: https://www.patreon.com/cperciva