Fix the POSIX-DYLD plugin to update the cached executable path after
attaching. Previously, the path was cached in DYLDRendezvous
constructor and not updated afterwards. This meant that if LLDB was
attaching to a process (e.g. via connecting to lldb-server), the code
stored the empty path before DidAttach() resolved it. The fix updates
the cached path in DidAttach().
This fixes a new instance of https://llvm.org/pr17880