Rename macos_build_flutter_assets.sh (#39353)

The name is very misleading at this point, since it does substantially
more than build flutter_assets. Name it more generically as a wrapper
for 'assemble' for macOS builds.

packages/flutter_tools/bin/macos_build_flutter_assets.sh → packages/flutter_tools/bin/macos_assemble.sh

@@ -4,7 +4,7 @@
 # found in the LICENSE file.
 
 # TODO(jonahwilliams): refactor this and xcode_backend.sh into one script
-# once macOS supports the same configuration as iOS.
+# once iOS is using 'assemble'.
 RunCommand() {
   if [[ -n "$VERBOSE_SCRIPT_LOGGING" ]]; then
     echo "♦ $*"