make u20 testing allow_failure

.gitlab-ci.yml

@@ -215,6 +215,7 @@ build-clang-8:
 # test for gcc
 test-u-20_04:
   extends: .test
+  allow_failure: true
   image: corsika/devel:u-20.04
   dependencies:
     - build-u-20_04
@@ -280,6 +281,7 @@ build_test-u-18_04:
 # build_test for gcc
 build_test-u-20_04:
   extends: .build_test
+  allow_failure: true
   image: corsika/devel:u-20.04
   dependencies:
     - config-u-20_04
@@ -401,6 +403,7 @@ build_test_example-u-18_04:
 # build_test_example for gcc
 build_test_example-u-20_04: 
   extends: .build_test_example
+  allow_failure: true
   image: corsika/devel:u-20.04
   dependencies:
     - config-u-20_04