Allow users to set multiple tags with the buildpacks task

[jromero]

Clone from tektoncd/catalog#423

Expected Behavior

With #415, the buildpacks task will be updated to use the creator binary. That allows users to add additional tags to the image by passing though -tag <TAG>. I would expect you to be able to pass TAGS to the buildpacks task, and it would work.

Actual Behavior

It doesn't work.

Additional Info

I spiked in working on this, but I found it non-trivial. Doing it in a good fashion (allowing users to say TAG: latest this-tag use-this-image) proves a bit difficult, because every one of those needs to be prefaced with -tag=), and when I tried to use it as an input to the create step, it gave me grief because it read it as an additional parameter to the task, instead of an argument.

We may want to look into using https://github.com/tektoncd/pipeline/blob/master/docs/tasks.md#emitting-results to properly convert the input string into one we can use as an arg to the task, though that would necessitate converting it from a Task to a Pipeline. See tektoncd/pipeline#1950 and tektoncd/pipeline#1978 for more details.