update pack --build-config flag to --config.

[lassebe]

Description

pack seems to have changed the flag from --build-config to --config at some point.

Checklist:

• [x] Run these changes with pack
• [ ] Make updates to CHANGELOG.md
  • Not sure if ^ is needed for such a minor change(?)

[danielleadams]

Dropping docs for reference: https://buildpacks.io/docs/operator-guide/create-a-builder/#2-create-builder