.gitlab-ci.yml: allow failure for release_valgrind_functional

see #4460: we currently have a bug in our test framework,
but otherwise all regulare valgrind tests are passing.

.gitlab-ci.yml

@@ -127,6 +127,7 @@ algo_valgrind_functional:
 
 release_valgrind_functional:
   <<: *valgrind_functional
+  allow_failure: true
   only:
     - /^feature-.*a.*\/release$/