Default value for the PLAN_CONTEXT on the build task should be "."

As the build task sets the `-s` parameter which states the src of the plan, the PLAN_CONTEXT is a relative path to this directory. This means that it should not be set as `$(Build.SourcesDirectory)` by default but to `.` which denotes the current dir.

This can be overridden in the task, but the default should be set appropriately