[LAr]Calorimeter: Move flake8 checking to build stage and enable checking in AthSimulation

Migrate the flake8 unit tests to using the POST_BUILD_CMD. Also make sure to always use the predefined ${ATLAS_FLAKE8} variable for the default configuration (additional ignore codes can be specified).

The whitespace changes in this MR as intentional to fix mixed tabs/spaces indentation.

This MR also enables flake8 checking for the AthSimulation project as some packages included in AthSimulation make use of custom flake8 checking, e.g.:

POST_BUILD_CMD ${ATLAS_FLAKE8} --extend-ignore=F401,F821 )

This breaks if ${ATLAS_FLAKE8} is undefined as the POST_BUILD_CMD will not be found. Alternatively, one could have

set( ATLAS_FLAKE8 true )

as the command true accepts any command line parameter. But since we already run flake8 checking in AthGeneration, probably good to do the same in AthSimulation.