build.ps1: set SDKROOT when testing #85178
                
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While adding a new LLDB test, I realized every LLDB test that runs Swift is disabled or xfailed. In the case of my test and at least one other one (see swiftlang/llvm-project#9620), this is due to
SDKROOTbeing unset. This manifests as a "unable to load standard library for target" error.When paired with a forthcoming change (either swiftlang/llvm-project#11717 or #85202), I'm hopeful that we can re-enable many of the disabled tests.