fix(dotnet): show nuget package dir not found log only when checking nuget packages

[DmitriyLewen]

Description

Currently we always show log that nuget package dir was not found :

➜  trivy -d image alpine
...
2024-07-19T14:31:41+06:00	DEBUG	[nuget] The nuget packages directory couldn't be found. License search 
...
2024-07-19T14:31:44+06:00	INFO	Number of language-specific files	num=0

But we need to show this log only when checking nuget files.

Related PRs

• feat(dotnet): add license support for NuGet #5217

Checklist

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• I've followed the conventions in the PR title.
• I've added tests that prove my fix is effective or that my feature works.
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