ci: Fix incremental coverage reports (#5552)

.github/workflows/compute-incremental-coverage.py

@@ -24,6 +24,9 @@ import zipfile
 
 # TODO(joeyparrish): Figure out how to get karma to output relative paths only.
 def StripGitDir(path):
+  # Convert Windows-style paths to Unix-style paths so we can read coverage
+  # data from Windows runners, too.
+  path = path.replace('\\', '/')
   # Strip the path to the git clone, leaving only the source path within the
   # repo.
   return re.sub(r'.*?/(lib|ui)/', r'\1/', path)