port: Fix broken VSIX build (#6124)

[github-actions[bot]]

The changes in Fix broken VSIX build (#6124) may need to be ported to maintain parity with microsoft/botbuilder-dotnet.

Fixes #minor

Description

The Directory.Build.props at the repo root gets used in building the VSIX templates solution, causing it to error.

Example: https://dev.azure.com/FuseLabs/SDK_v4/_build/results?buildId=287122&view=results

Specific Changes

Add an empty Directory.Build.props so the root copy is not used.

Please review and, if necessary, port the changes.