ci: don't run shard 2/12 ubsan without parallel.

3974806e5a added this:
    CI: Try not running group 2/10 UBSAN in parallel.
    
    It's being killed with signal 143, which means docker isn't happy; too much memory consumption?

But since we're now at 12 groups, that probably doesn't apply (it
might not have even before, in the two years since that commit since
so may things have been added).  And it caused this shard to take over
2 hours and timed out.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

.github/workflows/ci.yaml

@@ -553,7 +553,7 @@ jobs:
             PYTEST_OPTS: --test-group=1  --test-group-count=12
           - NAME: ASan/UBSan (02/12)
             GROUP: 2
-            PYTEST_OPTS: --test-group=2  --test-group-count=12 -n 1
+            PYTEST_OPTS: --test-group=2  --test-group-count=12
           - NAME: ASan/UBSan (03/12)
             GROUP: 3
             PYTEST_OPTS: --test-group=3  --test-group-count=12