[dotnet] Specify the dll as nuget content explicitly

[nvborisenko]

Description

Let's see how we can avoid [🐛 Bug]: Unpredictable nuget package structure while releasing a package in future.

Motivation and Context

This is a try to fix #13272

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• I have read the contributing document.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (e745a91) 58.04% compared to head (4b4ac2e) 58.05%.
Report is 8 commits behind head on trunk.

❗ Current head 4b4ac2e differs from pull request most recent head c0a59da. Consider uploading reports for the commit c0a59da to get more accurate results

Files	Patch %	Lines
py/selenium/webdriver/firefox/firefox_binary.py	50.00%	1 Missing ⚠️

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Additional details and impacted files

@@           Coverage Diff           @@
##            trunk   #13332   +/-   ##
=======================================
  Coverage   58.04%   58.05%           
=======================================
  Files          88       88           
  Lines        5337     5338    +1     
  Branches      224      224           
=======================================
+ Hits         3098     3099    +1     
  Misses       2015     2015           
  Partials      224      224           

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[nvborisenko]

Great! Now we control what exactly should go to the nuget packages. If some file is missing, we get build error.