pytest: reduce test_funding_v2_cancel_race nodes under CI.

```
[gw0] [ 24%] PASSED tests/test_misc.py::test_hsm_capabilities 
tests/test_connection.py::test_funding_cancel_race 
Error: Process completed with exit code 143.
```

Seems like 100 nodes is too many!

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

Author: rustyrussell

tests/test_connection.py

@@ -13,7 +13,7 @@
     mine_funding_to_announce, first_scid,
     CHANNEL_SIZE
 )
-from pyln.testing.utils import VALGRIND, EXPERIMENTAL_DUAL_FUND, FUNDAMOUNT, RUST
+from pyln.testing.utils import VALGRIND, EXPERIMENTAL_DUAL_FUND, FUNDAMOUNT, RUST, SLOW_MACHINE
 
 import os
 import pytest
@@ -1557,6 +1557,8 @@ def test_funding_v2_cancel_race(node_factory, bitcoind, executor):
 
     if VALGRIND:
         num = 5
+    elif SLOW_MACHINE:
+        num = 20
     else:
         num = 100