[4.0] SCons: Fix BoolVariable import for SCons 4.8.0+

[akien-mga]

See https://github.com/godotengine/godot/issues/94410 and https://stackoverflow.com/questions/78746956/error-name-boolvariable-is-not-defined-when-using-scons-on-godot-with-c for details.

Not needed in master as this was already changed by #1504.

Should be cherry-picked for 4.1 and 4.0, doesn't seem needed in 3.x/3.5.