cabal checks PATH before extra-prog-path

**Describe the bug**
When you have a program that's both on your `PATH` and in `extra-prog-path` you can't steer cabal to which one you want locally without modifying your `PATH`.

This is an issue with `pkg-config` on Windows because by default global installs of `msys2` (such as on appveyor) put the wrong version on your path.

```
$ cabal v2-build ...
```

**Expected behavior**
I expect cabal to use the paths I manually specified in `extra-prog-path` before the ones in `PATH`. This allows the use more control over which program they wanted used.