This change adds a `WorkingDirectory` field to `options.BuildOptions`,
but doesn't expose this as a CLI flag. The default zero value means the
current working directory. The value is used as the directory for
executing `go` tool commands.
When embedding ko in other tools, it is sometimes necessary to set the
working directory for executing the `go` tool, instead of assuming the
current process working directory.
An example of where this is required from Skaffold:
https://github.com/GoogleContainerTools/skaffold/tree/master/examples/microservices
In this example, the working directory doesn't contain either `go.mod`
or any Go files. The `skaffold.yaml` configuration file specifies
a `context` field for each image, which is the directory where the `go`
tool can find package information.
