Script compilation takes a dependency on host System.Private.CoreLib

**Version Used**:
Microsoft.CodeAnalysis.CSharp.Scripting 4.14.0

**Steps to Reproduce**:

```csharp
var code = "2+2";
string[] assemblyPaths = [/* Paths to .NET Standard 2.1 reference assemblies */];
var options = ScriptOptions.Default.WithReferences(assemblyPaths);
var script = CSharpScript.Create(code, options);
var compilation = script.GetCompilation();
var assemblyStream = new MemoryStream();
var emitResult = compilation.Emit(assemblyStream);
```

**Expected Behavior**:
`emitResult.Success == true`
The compiled assembly depends only on .NET Standard 2.1.

**Actual Behavior**:
`emitResult.Success == false`
`compilation.References` contains a reference to host `System.Private.CoreLib`.

**Notes**
Looks very similar to #8506, which is marked as fixed. The code from that bug report works correctly, probably the same approach would work for fixing this one as well.

Start with the CSharpScript.Create and GetCompilation repro in the issue, then compare its reference setup with the approach used for #8506. Investigate why compilation.References includes host System.Private.CoreLib when assemblyPaths contains .NET Standard 2.1 reference assemblies. Done means Emit succeeds and the compiled assembly depends only on .NET Standard 2.1.

Written by the indexing model from the issue text.