🐛 Fix logical error preventing force rebuild scans

[aaronleopold]

Fixes a small logical error that prevented the scan options from being considered when a modified timestamp could be extracted from a file on disk. This allows for the scan options to be considered but, as I mentioned on Discord, options will force a full rebuild of every book and corresponding metadata for the library. This is why I didn't expose it on the UI yet, that will be properly implemented by https://github.com/stumpapp/stump/tree/al/granular-scans.

I verified this fix by creating a library without koreader hashes enabled, enabling it, then running a scan with force_rebuild:

❯ curl http://localhost:10801/api/v1/media/7c760277-8b17-4c67-b2a8-72fe4c7008fb \
        -H "Authorization: Bearer stump_S36i9cffbt7_3y9uG3KDHfoVHnuk4JorjUL9FuWpbjRsi" | jq '{koreader_hash}'
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   753  100   753    0     0  29780      0 --:--:-- --:--:-- --:--:-- 30120
{
  "koreader_hash": null
}

❯ curl -X POST http://localhost:10801/api/v1/libraries/2e16160f-eb38-467d-96ce-2b039e73e8a9/scan \
        -H "Authorization: Bearer stump_S36i9cffbt7_3y9uG3KDHfoVHnuk4JorjUL9FuWpbjRsi" \
        -H "Content-Type: application/json" \
        -d '{"force_rebuild": true}'

❯ curl http://localhost:10801/api/v1/media/7c760277-8b17-4c67-b2a8-72fe4c7008fb \
        -H "Authorization: Bearer stump_S36i9cffbt7_3y9uG3KDHfoVHnuk4JorjUL9FuWpbjRsi" | jq '{koreader_hash}'
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   783  100   783    0     0  47043      0 --:--:-- --:--:-- --:--:-- 48937
{
  "koreader_hash": "803d9ee1501e9bc060fc1068d78008cc"
}

API keys are from my local development instance so it doesn't matter

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
core/src/filesystem/scanner/walk.rs	0.00%	4 Missing ⚠️

Files with missing lines	Coverage Δ
core/src/filesystem/scanner/walk.rs	0.00% <0.00%> (ø)