chore: require hashes when installing dependencies in owlbot postproc…

…essor job (googleapis#1691) (googleapis#1383)

* chore: install dependencies through requirements file
Source-Link: googleapis/synthtool@35f4cba
Post-Processor: gcr.io/cloud-devrel-public-resources/owlbot-java:latest@sha256:821ab7aba89af2c7907e29297bba024d4cd5366d0684e5eb463391cdf4edc9ee

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>
Co-authored-by: Diego Alonso Marquez Palacios <diegomarquezp@google.com>