build: cmake: generate deb packages under build/$<CONFIG>/debian

this follows the convention of configure.py, which puts
debian packages under build/$mode/debian.

Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>

dist/CMakeLists.txt

@@ -17,7 +17,7 @@ add_custom_command(
 add_custom_target(dist-server-deb
   COMMAND reloc/build_deb.sh
     --reloc-pkg ${unstripped_dist_pkg}
-    --builddir ${CMAKE_CURRENT_BINARY_DIR}
+    --builddir ${CMAKE_BINARY_DIR}/$<CONFIG>/debian
   DEPENDS ${unstripped_dist_pkg}
   WORKING_DIRECTORY ${CMAKE_SOURCE_DIR})
 add_custom_target(dist-server-rpm