ci: update release for new environment (googleapis#12493)

Install (non-interactively) Rebar3

Author: burkedavison

.kokoro/release.sh

@@ -24,6 +24,7 @@ export HEX_API_KEY=$(cat ${KOKORO_KEYSTORE_DIR}/73713_elixir_hex_apikey)
 
 if [[ -z "${FREEZE_RELEASES}" ]]; then
   mix local.hex --force
+  mix local.rebar --force
   mix deps.get
   mix google_apis.publish
 else