Doc: FIPS DS images come w/o microdnf

What does this MR do and why?

Clarify that FIPS-enabled images of Dependency Scanning don't have package managers such as dnf or microdnf. As a result, they don't allow users to install system packages at run-time in the before_script or elsewhere.

See gitlab-org/security-products/analyzers/gemnasium!886 (comment 2147343723)

Context is Change notice: UBI-based Secure scanner images ... (#471869 - closed)

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