Use NANO_NAME instead of a wildcard.

Since we already include  nano_env we have access to the
actual image name, so just use that.

This has two important effects.

1) If by chance you run make release in a dirty environment only
the images that are built from the current run will be included
in the release tarball.

2) No matter how the image name changes the script won't break due
to not chasing the name changes.