windows: kill rust-analyzer-proc-macro-srv before deleting stage0 directory

This fixes the following recurring error on windows:
```
Traceback (most recent call last):
  File "C:\Users\jyn\src\rust\x.py", line 29, in <module>
    bootstrap.main()
  File "C:\Users\jyn\src\rust\src\bootstrap\bootstrap.py", line 963, in main
    bootstrap(args)
  File "C:\Users\jyn\src\rust\src\bootstrap\bootstrap.py", line 927, in bootstrap
    build.download_toolchain()
  File "C:\Users\jyn\src\rust\src\bootstrap\bootstrap.py", line 437, in download_toolchain
    shutil.rmtree(bin_root)
  File "C:\Users\jyn\AppData\Local\Programs\Python\Python311\Lib\shutil.py", line 759, in rmtree
    return _rmtree_unsafe(path, onerror)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\jyn\AppData\Local\Programs\Python\Python311\Lib\shutil.py", line 617, in _rmtree_unsafe
    _rmtree_unsafe(fullname, onerror)
  File "C:\Users\jyn\AppData\Local\Programs\Python\Python311\Lib\shutil.py", line 622, in _rmtree_unsafe
    onerror(os.unlink, fullname, sys.exc_info())
  File "C:\Users\jyn\AppData\Local\Programs\Python\Python311\Lib\shutil.py", line 620, in _rmtree_unsafe
    os.unlink(fullname)
PermissionError: [WinError 5] Access is denied: 'C:\\Users\\jyn\\src\\rust\\build\\x86_64-pc-windows-msvc\\stage0\\bin\\rust-analyzer-proc-macro-srv.exe'
```

0 files changed, 0 insertions, 0 deletions