Issue with building solution filters (.slnf) using MsBuild.

[DmytroKukharuk]

Hi. It seems that MSBuildRunner populates arguments incorrectly when building a solution filter file. As the extension .slnf ends with an s, the GetArguments method identifies it as a .csproj file, but not as a solution. Therefore, the GetPlatformName method returns an incorrect result.

cake/src/Cake.Common/Tools/MSBuild/MSBuildRunner.cs

Line 98 in 124704e

bool isSolution = string.Equals(projectFile.GetExtension(), ".sln", StringComparison.OrdinalIgnoreCase);

As a result, the build fails with an error indicating that it cannot find the "Release|AnyCPU" configuration.
Error MSB4126: The specified solution configuration "Release|AnyCPU" is invalid. Please specify a valid solution configuration using the Configuration and Platform properties (e.g. MSBuild.exe Solution.sln /p:Configuration=Debug /p:Platform="Any CPU") or leave those properties blank to use the default solution configuration.

Proposed fix: Replace string.Equals with projectFile.GetExtension().StartsWith(".sln", StringComparison.OrdinalIgnoreCase).