Fix collect_steps script

PR #963 changed the root step name, now our script is failing.

Would be great to add it to CI, but I have high hopes that #961 is not
too far ahead.

Tested by manually running this script

scripts/collect_steps.py

@@ -12,7 +12,7 @@
 QUERY_SIZE = 100
 # attribution_window_seconds = 0 runs an optimized protocol, so 0 and anything larger
 ATTRIBUTION_WINDOW = [0, 86400]
-ROOT_STEP_PREFIX = "protocol/alloc::string::String::run-0"
+ROOT_STEP_PREFIX = "protocol/ipa_core::test_fixture::world::TestExecutionStep::iter0"
 BREAKDOWN_KEYS = 256
 USER_CAP = [8, 16, 32, 64, 128]
 SECURITY_MODEL = "semi-honest"