Set the image family next to OEM_PRELOADER_DIR

Choose the dev or presubmit image family in the same block that picks
OEM_PRELOADER_DIR, so the "acoshost" check lives in one place.

This also fixes the "Re-create this image with" message, which always
printed the presubmit family, even for dev builds.

TESTED=Ran main() locally with gcloud, docker and the build steps
stubbed. For dev and presubmit builds, the upload path and the image
create flags are unchanged; the message now prints host-acos-dev-<env>
for dev builds.

Reviewed-by: Maya Fleischer <mfleischer@google.com>
GitOrigin-RevId: 57fecd73c0aed1ebce80e045a7d6db20877a0e4d