don't fail if LDFLAGS env isn't set

[imjasonh]

Without this, if a .ko.yaml has:

builds:
- id: foo
  ldflags:
  - "{{ .Env.LDFLAGS }}"

(usually to set build-time VCS data like commit SHA, etc.)

If LDFLAGS isn't set, you'll get an error like:

Error: failed to publish images: error building "ko://blah/blah": template: argsTmpl:1:7: executing "argsTmpl" at <.Env.LDFLAGS>: map has no entry for key "LDFLAGS"

which is very annoying.

With this change, the build will succeed, without any additional -ldflags being set on the build.