builder.mk: Use `kubectl config view --flatten` to copy in DEV_KUBECONFIG
builder.mk: Use `kubectl config view --flatten` to copy in DEV_KUBECONFIG

This is the exact use-case that --flatten was created for.

(cherry picked from commit 6fe4523314ab8046216ff9f1df2ba0070779d106)

datawire/ambassador,datawire/ambassador,datawire/ambassador,datawire/ambassador,datawire/ambassador