Add a check task to the Makefile for required binaries

[seemiller]

Feature Request

As part of #409, we added a check-carvel task to verify that the imgpkg, kbld and vendir executables are present.

The task was not working properly on all workstations, so it was commented. Restore the task and make it work.

The check task should be a prerequisite for all of the package operations tasks.

Describe alternatives you've considered

Might be able to use a bash script if the Makefile approach doesn't work.