Install the official nRF Connect Toolchain package for your version of the SDK.
All the toolchain packages you can find through the nRF Connect for Desktop Toolchain Manager will be installable from this menu.
Contrary to the nRF Connect for Desktop Toolchain Manager, this mechanism will only install the toolchain package itself, and the SDK has to be installed separately.
The toolchain installer also supports configuring alternative toolchain indexes (through the nrf-connect.toolchainManager.indexURL configuration option), and installation path (through the nrf-connect.toolchainManager.installDirectory configuration option).