Clean `test_{runner,scalafmt_helper}`, add diagnostic env vars, and improve `test_cross_build` performance#1646

liucijus merged 2 commits intobazel-contrib:masterbazel-contrib/rules_scala:masterfrom

mbland:bzlmod-cleanup-test-runner-add-env-varsmbland/rules_scala:bzlmod-cleanup-test-runner-add-env-varsCopy head branch name to clipboard