chore: require hashes when installing dependencies in owlbot postprocessor job (#1691) (#1007)

* chore: install dependencies through requirements file
Source-Link: https://togithub.com/googleapis/synthtool/commit/35f4cbaf1295a726cb43fd4471129ec74b48e04e
Post-Processor: gcr.io/cloud-devrel-public-resources/owlbot-java:latest@sha256:821ab7aba89af2c7907e29297bba024d4cd5366d0684e5eb463391cdf4edc9ee

Author: gcf-owl-bot[bot]

.github/.OwlBot.lock.yaml

@@ -13,4 +13,4 @@
 # limitations under the License.
 docker:
   image: gcr.io/cloud-devrel-public-resources/owlbot-java:latest
-  digest: sha256:27d0a1232b3656f0610cc1a55beb157cb3b63b1ca8b3dfba0bcd8b9dfecda4a1
+  digest: sha256:27d0a1232b3656f0610cc1a55beb157cb3b63b1ca8b3dfba0bcd8b9dfecda4a1