Tested the customizer, the generated zip had:
    css/bootstrap.css:line 6: * Licensed under MIT (<%= _.pluck(pkg.licenses, "url").join(", ") %>)
Revised customizer generates:
    css/bootstrap.css:line 4: * Licensed under MIT (https://github.com/twbs/bootstrap/blob/master/LICENSE)

This fixes an error with `&:extend(.clearfix all)` when using the customizer.

follow-up to #11738