Visual Studio Build Tools prompt invocation is broken.

Hi,

I just noticed that lately and this seems to be happening since we moved to *Rez 2.16.0* last year but invoking the *Visual Studio Build Tools* prompt (2015 or 2017) is currently busted.

I noticed it because I did not have *cl* and *dumpbin* when invoking the prompt. Before when issuing:

```
rez-env vs_buildtools -- vcvars64
```

I was put in the *Visual Studio Build Tools 2017* prompt and had access to *cl*, etc... Now I actually need to do it staged:

```
rez-env vs_buildtools
```
Waiting for CMD prompt and then:

```
vcvars64
```

To properly spawn the *Visual Studio Build Tools 2017* prompt.

I'm not exactly sure what is the issue, probably usual Windows pain but I wanted to flag it.

Cheers,

Thomas