Backslash in property gets converted to forward slash on Linux

When a property contains backslashs in a project file, it will be converted to forward slashs when running with msbuild on Linux.

While this would be fine for paths (although one could just as well use forward slash in the project file since that works on both Windows and Linux), it causes problems when trying to set a regular expression in a property which requires a backslash to escape the next character.

### Steps to reproduce

Project file `bug.proj`:
```xml

\(([^]]+)\)

```

### Command line
```
msbuild /t:Build bug.proj
```

### Expected behavior

Outputs the line
```
Regex=\(([^]]+)\)
```

### Actual behavior

Outputs the line
```
Regex=/(([^]]+)/)
```

### Environment data

`msbuild /version` output: 15.6.0.0

Also happens with `xbuild` from older Mono versions.

When building with `dotnet build -v d bug.proj` the output is as expected.

OS Info:
Ubuntu 16.04.4 LTS