"cmd: not found" when building using mingw building a windows executable in linux
```
/home/hortinstein/.choosenim/toolchains/nim-1.6.10/lib/system/fatal.nim(54, 5) Error: unhandled exception: /home/hortinstein/.nimble/pkgs/nimterop-0.6.13/nimterop/build/shell.nim(90, 7) `not die` Command failed: 127
cmd: cmd /c mkdir  \home\hortinstein\.nimble\pkgs\monocypher-0.2.1\monocypher\build
result:
/bin/sh: 1: cmd: not found [AssertionDefect]
```

https://github.com/hortinstein/cross_compile/tree/master
run ```nimble buildwin``` to reproduce
