Build failed on Windows

[del-zhenwu]

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Checklist

1. I have searched related issues but cannot get the expected help.
2. I have read the FAQ documentation but cannot get the expected help.
3. The bug has not been fixed in the latest version.

Describe the bug
Windows is not supported when running Setup.py

19:37:50  powershell.exe : Traceback (most recent call last):
19:37:50  所在位置 C:\Workspace\Jenkins\workspace\mmdetection\build-on-windows\mmdetection@tmp\durable-961cdfce\powershellWrapper.ps1:3 字符: 1
19:37:50  + & powershell -NoProfile -NonInteractive -ExecutionPolicy Bypass -Comm ...
19:37:50  + ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
19:37:50      + CategoryInfo          : NotSpecified: (Traceback (most recent call last)::String) [], RemoteException
19:37:50      + FullyQualifiedErrorId : NativeCommandError
19:37:50   
19:37:50    File ".\setup.py", line 182, in <module>
19:37:50      add_mim_extension()
19:37:50    File ".\setup.py", line 169, in add_mim_extension
19:37:50      os.symlink(src_relpath, tar_path)
19:37:50  OSError: [WinError 1314] �ͻ���û���������Ȩ��: '..\\..\\tools' -> '.\\mmdet\\.mim\\tools'

Reproduction

1. What command or script did you run?

python setup.py develop

2. Did you make any modifications on the code or config? Did you understand what you have modified?
3. What dataset did you use?

Environment
python3.8

1. Please run python mmdet/utils/collect_env.py to collect necessary environment information and paste it here.
2. You may add addition that may be helpful for locating the problem, such as
   • How you installed PyTorch [e.g., pip, conda, source]
   • Other environment variables that may be related (such as $PATH, $LD_LIBRARY_PATH, $PYTHONPATH, etc.)

Error traceback
If applicable, paste the error trackback here.

A placeholder for trackback.

Bug fix
If you have already identified the reason, you can provide the information here. If you are willing to create a PR to fix it, please also leave a comment here and that would be much appreciated!