check_advisories: make the "bugs" field mandatory
check_advisories: make the "bugs" field mandatory

bedrock breaks when it's not present[1], and this would have caught the
mistake fixed by commit 858ccec2c "fix yaml format error"

[1] https://github.com/mozilla/bedrock/blob/747ee3e649afc1317e1bb6f0909696d3c65c33ca/bedrock/security/management/commands/update_security_advisories.py#L147

mozilla/foundation-security-advisories,mozilla/foundation-security-advisories