Fixes `OUTPUT_VARIABLE`, `ERROR_VARIABLE`.
vcpkg always wants logfiles. CMake (`execute_process`) doesn't send
stdout/stderr to files and variables at the same time, and even "the
precedence is not specified". So simultaneous output has to be handled
in `vcpkg_execute_required_process`.
At this occasion, port the arguments for stripping trailing whitespace.
`tensorflow-common` implicitly relied on the broken behavior. I chose
the minimal fix: drop the pointless calls, just set the empty value.
