How to add a new ROS core package without new build of all core package?

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I need to build the ROS from the source on Rasbian Buster (R-Pi 4). I followed the instructions from this site.

First I installed the desktop core version with these steps:

rosinstall_generator desktop --rosdistro melodic --deps --tar > melodic-desktop.rosinstall
wstool init -j8 src melodic-desktop-wet.rosinstall
wstool update -j 4 -t src

It works. However, now I need several other packages, such as tf2_eigen. I check the instruction from https://wiki.ros.org/melodic/Installa.... Then, I execute

rosinstall_generator desktop_full --rosdistro melodic --deps --tar > melodic-desktop-full.rosinstall

to see the entries of the tf2_eigen. I inserted the tf2_eigen entry to the file melodic-desktop.rosinstall, that I created for the first time.

Then, I did

wstool merge -t src  melodic-desktop.rosinstall

to merge the new entry, and wstool update -j 4 -t src.

However, after I run this command

sudo ./src/catkin/bin/catkin_make_isolated --install -DCMAKE_BUILD_TYPE=Release --install-space /opt/ros/melodic -j2

to build the tf2_eigen, the command catkin_make_isolated build everything again in the src folder. I costs me two hours to build all the core packages again. How can I build the tf2_eigen only, without building everything again?