buildx/controller/build
Justin Chadwell a7e471b7b3 controller: only capture the result when invoked through a controller
This ensures that the code used to capture and evaluated a result is
only executed when built through the controller. Otherwise, no build
result should be recorded.

This ensures that new code added to capture and store the build result
for debugging isn't used when BUILDX_EXPERIMENTAL is not set.

Signed-off-by: Justin Chadwell <me@jedevc.com>
..
build.go controller: only capture the result when invoked through a controller
print.go controller: move controllers out of commands into separate package