Chaining PlugInstall and PlugUpdate breaks

I tried to execute `PlugInstall | PlugUpdate` and had that mapped to a command, because I didn't know that PlugUpdate would also install missing plugins.
After adding a new plugin, these two commands apparently break each other and throw everything full of errors.

This is not really a valid usecase, but still thought I might report it.

------------------------------

```
NVIM v0.4.3
Build type: Release
LuaJIT 2.1.0-beta3
Compilation: /usr/bin/cc -g -O2 -fdebug-prefix-map=/build/neovim-U5Ub0w/neovim-0.4.3=. -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=1 -DDISABLE_LOG -Wdate-time -D_FORTIF
Y_SOURCE=1 -O2 -DNDEBUG -DMIN_LOG_LEVEL=3 -Wall -Wextra -pedantic -Wno-unused-parameter -Wstrict-prototypes -std=gnu99 -Wshadow -Wconversion -Wmissing-prototypes -Wimplicit-fallthrough -Wvla -fstack-protector-str
ong -fdiagnostics-color=always -DINCLUDE_GENERATED_DECLARATIONS -D_GNU_SOURCE -DNVIM_MSGPACK_HAS_FLOAT32 -DNVIM_UNIBI_HAS_VAR_FROM -I/build/neovim-U5Ub0w/neovim-0.4.3/build/config -I/build/neovim-U5Ub0w/neovim-0.
4.3/src -I/usr/include -I/usr/include/lua5.1 -I/build/neovim-U5Ub0w/neovim-0.4.3/build/src/nvim/auto -I/build/neovim-U5Ub0w/neovim-0.4.3/build/include
Compiled by team+vim@tracker.debian.org
```

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