testsuite: Declare bkpcabal08 as fragile

Due to spurious output changes described in #23648.

testsuite/tests/backpack/cabal/bkpcabal08/all.T

@@ -6,6 +6,7 @@ else:
 test('bkpcabal08',
      [extra_files(['p', 'q', 'impl', 'bkpcabal08.cabal', 'Setup.hs', 'R.hs']),
       js_broken(22351), 
+      fragile(23648),
       normalise_slashes,
       normalise_version('bkpcabal08')],
      run_command,