bk: SoftFail() means any exit code is ok

Created by: jhchabran

Prior to this pr, the buildkite.SoftFail(int...) helper did not support using the * form of the soft fail attribute. Now, it's possible to do so, by simply using the zero arguments form of the buildkite.SoftFail() function.

Test plan

Tested here: https://buildkite.com/sourcegraph/sourcegraph/builds/164165#01824927-af32-4411-87ed-f513a6727334