Enable testing on Heidelberg runner

.gitlab-ci.yml

@@ -22,11 +22,17 @@ before_script:
 git--gcc:
   image: registry.dune-project.org/joe/dune-perftool-ci/perftool-base:git
   script: duneci-standard-test
+  tags:
+    - cores:20
 
 git--clang:
   image: registry.dune-project.org/joe/dune-perftool-ci/perftool-base:git
   script: duneci-standard-test --opts=/duneci/opts.clang
+  tags:
+    - cores:20
 
 git--pacxx:
   image: registry.dune-project.org/joe/dune-perftool-ci/pacxx-perftool-base:git
   script: duneci-standard-test --opts=/duneci/opts.pacxx
+  tags:
+    - cores:20