Allow using `go test -c` as build command

We often use `go test -c` to create binaries that need to run in a cluster to run their tests inside kubernetes. Currently `ko` only supports `go build` as a build command, so for these tests there is currently no way to be applied using `ko`.

I'd be really glad if a flag would allow me to change the build command.

Start by tracing how ko currently invokes go build and how build-command options are exposed. Check how a selectable command would support go test -c for binaries used in Kubernetes tests, then add coverage for the alternate command and verify the existing build path remains unchanged.

Written by the indexing model from the issue text.