Change default output directory for Frosting to repository root

[pascalberger]

Cake.Frosting.Issues.Recipe expects to run in a sub directory of the project and have the build directory set as working directory (BuildRootDirectory``= ./, ProjectRootDirectory=../`).

Output directory for Cake.Frosting.Issues.Recipe is currently BuildArtifacts relative to the working directory which will result in a sub directory BuildArtifacts below the build directory and not the project directory.