Remove invalid `local` from macos_assemble.sh (#58030)

When you run this, bash complains that `local` is only valid in functions. Gets rid of that extraneous warning.
https://dart.googlesource.com/external/github.com/flutter/flutter/+/98ce4a5dfef7af8fe4e0a9b93670ad028329c08a
1 file changed
tree: eb90d6ccc5ee8597dcc4d7b9ba4357d5119e9a9b
  1. commits.json