workflows/tests: set HOMEBREW_LIVECHECK_AUTOBUMP

We recently modified the behavior of `brew livecheck` to skip
packages in the tap's autobump list, so this change is causing
autobumped formulae to fail the `brew livecheck` part of CI testing.
This adds the `HOMEBREW_LIVECHECK_AUTOBUMP` ENV variable, which
will ensure that `brew livecheck` doesn't skip autobumped formulae
(without manually passing the `--autobump` flag).

.github/workflows/tests.yml

@@ -241,6 +241,7 @@ jobs:
             --added-formulae="$ADDED_FORMULAE" \
             --deleted-formulae="$DELETED_FORMULAE"
         env:
+          HOMEBREW_LIVECHECK_AUTOBUMP: 1
           TEST_BOT_FORMULAE_ARGS: ${{ needs.setup_tests.outputs.test-bot-formulae-args }}
           TESTING_FORMULAE: ${{ needs.formulae_detect.outputs.testing_formulae }}
           ADDED_FORMULAE: ${{ needs.formulae_detect.outputs.added_formulae }}