$(SolutionDir) ignored for shared projects

**Operating System**: Windows

**DocFX Version Used**: 2.56.0
.
**Template used**: default

**Steps to Reproduce**:

In the csproj file I have

```xml

```

And docfx says I have no items in my csproj files and no api docs generated. I use the SDK csproj format.

**Expected Behavior**:

Generate API docs when I use $ variable instead of relative path.

**Actual Behavior**:

API docs are not generated.