[BUGFIX] Re-introduce `true` default value for button radius mixin

This re-introduces the correct change made in #4454 which was
accidentally reverted with 9d029fcde921efca9a7b502956de9aaa0a7e12bb:

The `button-style` mixin must have a default value of `true` in order
for global radius settings taking effect.
Merge pull request #5271 from afoeder/master

Re-introduce `true` default value for button radius mixin