[release/6.0] Fix MSBuild property name for MicrosoftBuildTasksCoreVersion (#37411)

- backport of #37404, cherry-pick of 8b095602

Fix MSBuild property name for MicrosoftBuildTasksCoreVersion (#37404)

  * There was a missing 's', which caused source-build to use a pre-built when building.

Co-authored-by: Eric Erhardt <eric.erhardt@microsoft.com>