Including multiple semicolon-separated strings removes slash on Linux

### Steps to reproduce

Project file a.proj:
```xml

<_Value0>/tmp/RoslynTests/5b60c3b6-f6b5-431b-a782-fc3f8a5eea90/
<_Value1>https://raw.githubusercontent.com

```

Command line
```
> dotnet msbuild a.proj
```

### Expected behavior

Content of Result1.txt and Result2.txt is the same:

```
/tmp/RoslynTests/5b60c3b6-f6b5-431b-a782-fc3f8a5eea90/
https://raw.githubusercontent.com
```

### Actual behavior
Result1.txt:
```
/tmp/RoslynTests/5b60c3b6-f6b5-431b-a782-fc3f8a5eea90/
https:/raw.githubusercontent.com
```
Result2.txt:
```
/tmp/RoslynTests/5b60c3b6-f6b5-431b-a782-fc3f8a5eea90/
https://raw.githubusercontent.com
```

### Environment data
`msbuild /version` output:

```
Microsoft (R) Build Engine version 15.7.177.53362 for .NET Core
Copyright (C) Microsoft Corporation. All rights reserved.
```

OS info:
- Fails on Ubuntu.
- Works on Windows