[LLDB][PDB] Require Windows for for testing PDB plugin-selection again (#158559)
Amends #158284 and fixes the failure on `lldb-remote-linux-win` from
https://github.com/llvm/llvm-project/pull/158284#issuecomment-3290154510.
That builder is configured with the DIA SDK but builds for Linux, so the
debug information will be DWARF, not PDB.