You are already doing this for the framework.
See docs. The used compiler is still
So just lookup available versions at bintray and e.g. say
[env:d1_mini]
platform = espressif8266
board = d1_mini
framework = arduino
platform_packages =
framework-arduinoespressif8266 @ https://github.com/esp8266/Arduino.git
toolchain-xtensa@2.100100.200706
to give you GCC 10.1.0.