chore: require hashes when installing dependencies in owlbot postprocessor job (#1691) (#2138)

* 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 <[email protected]>

Author: gcf-owl-bot[bot]

.github/.OwlBot.lock.yaml

@@ -14,3 +14,4 @@
 docker:
   image: gcr.io/cloud-devrel-public-resources/owlbot-java:latest
   digest: sha256:a57d2ea6d1a77aa96c17ad0850b779ec6295f88b6c1da3d214b2095d140a2066
+