pw_presubmit: Correct coverage ref

Based on b/261779031#comment12.

Bug: b/279161371
Change-Id: I8b8e2d53117da677e477c52fe891df642b64db44
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/179452
Commit-Queue: Ted Pudlik <tpudlik@google.com>
Reviewed-by: Rob Mohr <mohrr@google.com>

pw_presubmit/py/pw_presubmit/pigweed_presubmit.py

@@ -218,7 +218,7 @@ def _gn_combined_build_check_targets() -> Sequence[str]:
         project='codesearch',
         trace_type='LLVM',
         trim_prefix='/b/s/w/ir/x/w/co',
-        ref='main',
+        ref='refs/heads/main',
         source='infra:main',
         owner='pigweed-infra@google.com',
         bug_component='503634',