T8761 failing for ext-interp

Hi Simon, Phab tells us that T8761 is failing for the ext-interp way, as of d2006d050e7a9111c0c448d6262f8994ef5761b7 (Run all TH tests with -fexternal-interpreter (#12219)). See https://phabricator.haskell.org/B10229 Thanks, Richard