Error when I try to install plugins on Windows

I am using vim-plug to manage my plugins both on Windows and Linux. But on Windows, I encounter errors sometimes when I try to install new plugins using `PlugInstall` command. The error is shown blow:

```
|| Error detected while processing function
C:\Users\Administrator\AppData\Local\nvim\autoload\plug.vim|604| 2_install[1]
C:\Users\Administrator\AppData\Local\nvim\autoload\plug.vim|1015| 2_update_impl[58]
C:\Users\Administrator\AppData\Local\nvim\autoload\plug.vim|327| 2_git_version_requirement[2]
C:\Users\Administrator\AppData\Local\nvim\autoload\plug.vim|2039| 2_system[6]
|| E482: Can't open file C:\Users\ADMINI~1\AppData\Local\Temp\nvimBCqYQf\10.bat for writing: no such file or directory
|| Error detected while processing function
C:\Users\Administrator\AppData\Local\nvim\autoload\plug.vim|604| 2_install[1]
C:\Users\Administrator\AppData\Local\nvim\autoload\plug.vim|1015| 2_update_impl[58]
C:\Users\Administrator\AppData\Local\nvim\autoload\plug.vim|327| 2_git_version_requirement[2]
|| E171: Missing :endif
```

This error seems random and only appear after I have used nvim-qt for some time. If I quit nvim-qt and restart, I can use `PlugInstall` without any error.

## version info

```
NVIM v0.4.0-819-g66d127957
Build type: RelWithDebInfo
LuaJIT 2.0.5
Compilation: C:/Program Files (x86)/Microsoft Visual Studio/2017/Community/VC/Tools/MSVC/14.16.27023/bin/Hostx86/x64/cl.exe /DWIN32 /D_WINDOWS /W3 /MD /Zi /O2 /Ob1 /DNDEBUG -DMIN_LOG_LEVEL=3 /W3 -D_CRT_SECURE_NO_WARNINGS -D_CRT_NONSTDC_NO_DEPRECATE -DWIN32 -D_WIN32_WINNT=0x0600 -DINCLUDE_GENERATED_DECLARATIONS -DNVIM_MSGPACK_HAS_FLOAT32 -DNVIM_UNIBI_HAS_VAR_FROM -IC:/projects/neovim/build/config -IC:/projects/neovim/src -IC:/projects/nvim-deps/usr/include -IC:/projects/neovim/build/src/nvim/auto -IC:/projects/neovim/build/include
Compiled by appveyor@APPVYR-WIN

Features: -acl +iconv +tui
See ":help feature-compile"

system vimrc file: "$VIM\sysinit.vim"
fall-back for $VIM: "C:/Program Files/nvim/share/nvim"

Run :checkhealth for more info
```

- Type:
- [x] Bug
- [ ] Enhancement
- [ ] Feature Request
- [ ] Question
- OS:
- [ ] All/Other
- [ ] Linux
- [ ] OS X
- [x] Windows
- Vim:
- [ ] Terminal Vim
- [ ] GVim
- [x] Neovim