fix: add --branch flags to repo clone to support branch checkout (#1940)

Author: ldetmer

infra/prod/generate.yaml

@@ -21,6 +21,7 @@ steps:
     args:
       - 'clone'
       - '--depth=1'
+      - '--branch=$_BRANCH'
       - $_FULL_REPOSITORY
   - name: gcr.io/cloud-builders/git
     id: configure-language-repo-name

infra/prod/stage-release.yaml

@@ -22,6 +22,8 @@ steps:
     waitFor: ['-']
     args:
       - 'clone'
+      - '--single-branch'
+      - '--branch=$_BRANCH'
       - $_FULL_REPOSITORY
   - name: gcr.io/cloud-builders/git
     id: configure-language-repo-name