How to reload systemd user environment variables?

I have some env variables set under ~/.config/environment.d. They work fine.

What’s not clear is how to reload my user environment whenever I change these variables. Logging out is not enough, rebooting seems to be the only way.

systemctl --user set-environment VARIABLE=xxx seems to work temporarily, but log out and you’ll lose the changes. What I’m asking for is a single, consistent way of reloading everything under ~/.config/environment.d short of rebooting the entire system.

See also: systemd/User - ArchWiki