(GH-18) Explicitly set the BHBuildOutput variable

[pauby]

Description

Explicitly set the BHBuildOutput variable.

Related Issue

#18

Motivation and Context

This allows the environment variable to be used outside of Invoke-Build (I am using it inside Pester tests for example).

How Has This Been Tested?

This has only been tested in an Invoke-Build environment. But the tasks that have been amended are obviously for psake anyway,

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[devblackops]

LGTM. Thanks!