conf:  .path with var and parentDir results in wrong path

### Nim Version

2.3.1
2.*

### Description

e.g.,

If in nim.cfg, using

```
gcc.path = $nim/../../xxx
```

Then it just always became ../xxx




### Current Output

```text

```

### Expected Output

```text

```

### Known Workarounds

_No response_

### Additional Information

It's because in extccomp.nim, joinPath was called before strutils.\`%\`