[windows] `@swift-ci test llvm` command does not work on Windows

[charles-zablit]

The @swift-ci test llvm command does not work on Windows.

We should edit the CI script or utils/build.ps1 to implement it.

[charles-zablit]

cc @compnerd

[compnerd]

build.ps1 has nothing specific for the CI support, this would be in the CI scripts.