workflows/sorbet: fix pull request testing

.github/workflows/sorbet.yml

@@ -47,6 +47,8 @@ jobs:
         id: update
         working-directory: ${{ steps.set-up-homebrew.outputs.repository-path }}
         run: |
+          if [[ "${GITHUB_EVENT_NAME}" != "pull_request" ]]
+          then
             git fetch origin
 
             BRANCH="sorbet-files-update"
@@ -59,6 +61,7 @@ jobs:
             else
               git checkout --no-track -B "${BRANCH}" origin/master
             fi
+          fi
 
           brew typecheck --update --suggest-typed