ConfigStep: don't fail copying over existing hook

[derrickstolee]

I was perusing my logs when I found the following error:

System.IO.IOException: The file 'C:\\_git\\scalar\\src\\.git\\hooks\\query-watchman' already exists.
   at System.IO.FileSystem.CopyFile(String sourceFullPath, String destFullPath, Boolean overwrite)
   at System.IO.File.Copy(String sourceFileName, String destFileName, Boolean overwrite)
   at Scalar.Common.FileSystem.PhysicalFileSystem.CopyFile(String sourcePath, String destinationPath, Boolean overwrite) in E:\\A\\_work\\23\\s\\Scalar.Common\\FileSystem\\PhysicalFileSystem.cs:line 75
   at Scalar.Common.Maintenance.ConfigStep.ConfigureWatchmanIntegration() in E:\\A\\_work\\23\\s\\Scalar.Common\\Maintenance\\ConfigStep.cs:line 250

"ErrorMessage":"Failed to configure Watchman integration: The file
'C:\\_git\\scalar\\src\\.git\\hooks\\query-watchman' already exists.

Avoid this issue by overwriting the existing hook. This should work perfectly on Mac, and could still get an exception if someone has a read handle open on Windows, but should be rare. In that case, having the exception in the logs is still useful. (Cc: @kewillford)