Don't fail builds when GIT_API_KEY is not available (on PRs).

We don't need GIT_API_KEY to clone the assets branch, so don't require it.

gh-assets-clone.sh

@@ -3,7 +3,6 @@
 # GIT_API_KEY.
 [ ! -z "$GIT_ASSETS_BRANCH" ] || exit 1
-[ ! -z "$GIT_API_KEY" ] || exit 1
 setup_git() {
   git config --global user.email "travis@travis-ci.org" || exit 1