credentials should be a TaskKey to make it possible to compute its value from other settings

The use case I have is that I need to dynamically compute the credentials setting from
other settings I have in my build (i.e. I have several settings that look up for
AWS credentials in various places). Having the credentials setting as a SettingKey
makes it impossible to calculate its value by mapping the value of other SettingKey(s)
or TaskKey(s).