Having relative paths on $PATH breaks stack (with system-ghc)

### Steps to reproduce

With a relative path in `$PATH` (e.g. [for this trick](https://thoughtbot.com/blog/git-safe))

```
% echo $PATH
.git/safe/../../bin:/home/greg/.local/bin:/bin:/usr/local/sbin:/usr/local/bin:/sbin:/usr/sbin:/usr/bin
```

try to build something with system GHC.

### Expected

Should work :)

### Actual

```
InvalidAbsFile "/path/to/current/dir/.git/safe/../../bin/ghc-8.6.5"
```

### Stack version

```
% stack --version
Version 2.1.3.1 aarch64
```

### Method of installation

* FreeBSD pkg