Add a Valgrind GitHub Actions workflow#1104

wilfwilson wants to merge 2 commits intosemigroups:stable-5.5semigroups/Semigroups:stable-5.5from

wilfwilson:add-valgrind-ci-workflowwilfwilson/Semigroups:add-valgrind-ci-workflowCopy head branch name to clipboard