ci: fix the propagation of TEST_USER_GITHUB

A typo in the reference to the repository-level `TEST_USER_GITHUB`
variable was causing export tests under //bridge/github to be skipped.
This change resolves this issue.

Change-Id: Ia1ad61e54e305bb073efc6853738d3eed81d576c

.github/workflows/build-and-test.yml

@@ -29,7 +29,7 @@ jobs:
       - name: Test
         run: make test
         env:
-          GITHUB_USER: ${{ env.TEST_USER_GITHUB }}
+          GITHUB_USER: ${{ vars.TEST_USER_GITHUB }}
           GITHUB_TOKEN: ${{ secrets.TEST_TOKEN_GITHUB }}
           GITHUB_TOKEN_PRIVATE: ${{ secrets._GITHUB_TOKEN_PRIVATE }}
           GITHUB_TOKEN_PUBLIC: ${{ secrets._GITHUB_TOKEN_PUBLIC }}