%ComSpec% ignored by vim-plug

On Windows, the standard environment variable %ComSpec% should be used to overwrite the command processor. However, vim-plug hard-codes the use of cmd.exe. This means that if cmd.exe fails (which can happen if it is blocked) jobs will always fail, giving the error `Job failed to start`.

The following is the line where it is hard coded, at line 1452.
```vimscript
let argv = s:is_win ? ['cmd', '/s', '/c', '"'.cmd.'"'] : ['sh', '-c', cmd]
let jid = job_start(s:is_win ? join(argv, ' ') : argv, {
```

Replacing cmd with what I have set in %ComSpec% allows the jobs to start, but this solution has major shortcomings, like breaking any time vim-plug should be updated. What would be better is to check the value of ComSpec first, and falling back to cmd if it is an invalid value.