[llvm-cov] Attempt to appease Windows bots

They appear to reject r275640 because stdin is held open during an
ExecuteAndWait in which it's redirected:

  http://lab.llvm.org:8011/builders/llvm-clang-lld-x86_64-scei-ps4-windows10pro-fast/builds/8390

llvm-svn: 275642