Use full 40-char commit hash in find-upstream-image.sh

Change find-upstream-image.sh to use --full flag when calling
find-last-source-commit.sh, so UPSTREAM_COMMIT contains the full
40-character commit hash instead of the abbreviated 8-character version.

This allows downstream repos to use UPSTREAM_COMMIT for cache keys
(which need full hashes) while UPSTREAM_TAG remains abbreviated for
Docker image tags.

The get-cached-image.sh script already handles abbreviating long
commits for tags, so this change is backwards compatible.