Consider all providers and edge cases when determining earliest data timestamp

[wagoodman]

This is an additional fix on top of #440 -- the original PR did not consider if NVD was the first provider on the list.

Example run:

[0002] DEBUG state validated for all providers
[0002] TRACE not considering data age for provider provider=nvd
[0002] DEBUG earliest data timestamp timestamp=2024-11-25 01:31:56.419535 +0000 +0000
[0002]  INFO building database build-directory=./build providers=[nvd alpine amazon chainguard debian github mariner oracle rhel sles ubuntu wolfi] schema=5