fix newest-file selection for offline cache

[Strum355]

(Issue discovered while using https://github.com/chainguard-dev/rules_apko)

Given the following keyrings:

- https://packages.sgdev.org/sourcegraph-melange-prod.rsa.pub
- https://packages.sgdev.org/chainguard/chainguard-enterprise.rsa.pub

the following directory structure is created:

cache_wolfi_base_apko/https%3A%2F%2Fpackages.sgdev.org%2F/
├─ chainguard/
│  ├─ chainguard-enterprise.rsa.pub
├─ sourcegraph-melange-prod.rsa.pub

This is fine when looking for the public key file for https://packages.sgdev.org/chainguard/chainguard-enterprise.rsa.pub, but may fail for https://packages.sgdev.org/sourcegraph-melange-prod.rsa.pub if the directory chainguard/ has a newer timestamp than the public key file (I was not able to reproduce this locally on darwin, but reliably on linux CI machines).

This PR addresses this by not selecting directories as possible "newest" dir entries.

[imjasonh]

Hey sorry for missing this PR! 😭

In general the fix seems good, thanks for the thorough explanation of the problem. If there's any way you could add a test to demonstrate the behavior and the fix, that'd be helpful both for a.) confirming I understand the issue, and b.) ensuring future changes don't regress this fix unexpectedly.

Thanks again, and I think we'll be able to get this in once we have a simple test.

[Strum355]

Hey sorry for missing this PR! 😭

In general the fix seems good, thanks for the thorough explanation of the problem. If there's any way you could add a test to demonstrate the behavior and the fix, that'd be helpful both for a.) confirming I understand the issue, and b.) ensuring future changes don't regress this fix unexpectedly.

Thanks again, and I think we'll be able to get this in once we have a simple test.

Sorry for the long turnaround time. I've pushed a test case for this. LMK if this suffices