Default target platform for 'buck2 run'

[dmezh]

When you buck2 run today, there's nothing to tell buck2 that you're probably trying to build for the host platform as the target. It would be really sweet if there could be an option (.buckconfig?) to specify a default target platform specifically for the target being used in 'buck2 run' for its RunInfo.

[alexlian]

Configuration Modifiers will be able to do this (soon enough), though we need to get host-conditional modifiers in before this will be possible.

https://buck2.build/docs/api/build/#set_modifiers