Default `SIGN_IMAGES` variable to `false` (#225)
Default `SIGN_IMAGES` variable to `false` (#225)

* Default `SIGN_IMAGES` variable to `false`

nightly release failed with:
```
./hack/../vendor/knative.dev/hack/release.sh: line 314: SIGN_IMAGES: unbound variable
```

Signed-off-by: Pierangelo Di Pilato <bf2f29ea09b3cb50cb8a51bbebdb9b3cef9d9af1@redhat.com>

* Update release.sh

Signed-off-by: Pierangelo Di Pilato <bf2f29ea09b3cb50cb8a51bbebdb9b3cef9d9af1@redhat.com>
knative/hack,knative/hack