Add Pester 6 unit tests for Initialize-PSBuild.

Scenarios to cover:

• Environment variables and build properties are set correctly
• Path resolution works on both PS 5.1 and PS 7 (see INIT task throws an exception in Windows PowerShell #53)
• Mock Set-BuildEnvironment to avoid filesystem side effects

No fixture module needed. Target Pester 6.0.0 with classic Should syntax — see notes on #17. Parent issue: #17