.github/workflows/ci-conda-known-test-failures.json: Add failure seen in https://github.com/sagemath/sage/actions/runs/6835143781/job/18588599649\#step:11:16939

Author: Matthias Koeppe

.github/workflows/ci-conda-known-test-failures.json

@@ -14,6 +14,9 @@
     "sage.libs.singular.singular": {
         "failed": true
     },
+    "sage.matrix.matrix2": {
+        "failed": true
+    },
     "sage.misc.lazy_import": {
         "failed": true
     },