Remove working directory for debugserver code signing target

Summary:
LLVM puts output binaries in
`${CMAKE_CURRENT_BINARY_DIR}/${CMAKE_CFG_INTDIR}/bin`. This chunk of
code left out the `${CMAKE_CFG_INTDIR}`. Ultimately, the code signing target
does not need a working directory anyways, so it's safe to just remove it.

Reviewers: sas, xiaobai, beanz

Reviewed By: beanz

Subscribers: mgorny

Differential Revision: https://reviews.llvm.org/D53959

llvm-svn: 346281