ctest: Set close-on-exec for jobserver FIFO

Set the close-on-exec flag to prevent the jobserver FIFO file
descriptor from leaking to subprocesses spawned by `ctest`.

This leak is usually harmless, but can cause `make test -jN` to hang
at the end of the build if a test leaves a background process running
with the jobserver FIFO file descriptor open.

Fixes: #26027

Source/CTest/cmUVJobServerClient.cxx

@@ -306,6 +306,10 @@ void ImplPosix::ConnectFIFO(const char* path)
   if (fd < 0) {
     return;
   }
+  if (fcntl(fd, F_SETFD, FD_CLOEXEC) == -1) {
+    close(fd);
+    return;
+  }
 
   cm::uv_pipe_ptr connFIFO;
   connFIFO.init(this->Loop, 0, this);