dotnet restore fails only on Linux when Sdk="Microsoft.Net.Sdk"

_From @danpere on August 28, 2017 18:9_

## Steps to reproduce
1. Create a `.csproj` file in the new format that starts
```xml

```
and has NuGet packages referenced. (Note the casing: [the docs](https://docs.microsoft.com/en-us/dotnet/core/tools/csproj) say the `Sdk` should be "`Microsoft.NET.Sdk`" with `NET` in all caps.)

2. Run `dotnet restore` on Windows.
3. Run `dotnet restore` on Linux.

## Expected behavior
`dotnet restore` fails or succeeds on both platforms.

## Actual behavior
`dotnet restore` successfully restores packages on Windows. On Linux it displays
```
Foo.csproj : error MSB4236: The SDK 'Microsoft.Net.Sdk' specified could not be found.
```

## Environment data
`dotnet --info` output:
```
.NET Command Line Tools (2.0.0)

Product Information:
Version: 2.0.0
Commit SHA-1 hash: cdcd1928c9

Runtime Environment:
OS Name: ubuntu
OS Version: 14.04
OS Platform: Linux
RID: ubuntu.14.04-x64
Base Path: /usr/share/dotnet/sdk/2.0.0/

Microsoft .NET Core Shared Framework Host

Version : 2.0.0
Build : e8b8861ac7faf042c87a5c2f9f2d04c98b69f28d
```

_Copied from original issue: dotnet/cli#7541_