mingw unpacking fails with custom choosenimDir
Apparently, `choosenim` doesn't like when `--choosenimDir` points to another drive if it's different from `TEMP`:

```cmd
set TEMP=c:\windows\temp
choosenim --firstInstall --choosenimDir:d:\nim\choosenim --debug

>      Error: Unable to extract. Error was 'Access is denied.
>        ... Additional info: "(\"C:\\\\WINDOWS\\\\TEMP\\\\nimarchive-3489535509\\\\mingw64\\\\bin\", \"d:\\\\nim\\\\choosenim\\\\toolchains\\\\mingw64\\\\bin\")"'.
```

On the same drive it's it works fine:
```cmd
set TEMP=d:\nim\temp
mkdir %TEMP%
choosenim --firstInstall --choosenimDir:d:\nim\choosenim --debug
...
   Switched to Nim 1.4.4
```

Although it seems that it's a pain anyway to specify a custom directory for `choosenim` and `nimble`:
 * `choosenim` doesn't store the specified directory and always wants it in a command line
 * `nimble` uses its own [`nimble.ini`](https://github.com/nim-lang/nimble#configuration) and I have no idea how well it plays with `choosenim`

Sigh.