CI: allow the failure for the test_opam_test job

Indeed, it may fail if an ADT changes in lutils or rdbg-plugin, since
this repo can only be updated if the whole pipeline succeeds (to be
able te get the release number). The idea is to rerun this job manually
when it is the case.
2 jobs for 2.68.0 in 8 minutes and 54 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #78186
build

00:04:46

 
  Test
passed #78187
test_opam_test

00:04:07