<Sdk> element does not respect Conditions

### Steps to reproduce

Project file
```xml

```

Command line
```
msbuild
```

### Expected behavior
Successful build

### Actual behavior
Error similar to:
```
E:\tmp\test\Test.proj : error : C:\Program Files\dotnet\sdk\2.1.601\Sdks\DoesNotExist\Sdk not found. Check that a recent enough .NET Core SDK is installed and/or increase the version specified in global.json.
```

### Environment data
`msbuild /version` output:
```
Microsoft (R) Build Engine version 16.0.450+ga8dc7f1d34 for .NET Framework
Copyright (C) Microsoft Corporation. All rights reserved.

16.0.450.56488
```
OS info: Win10