ci: fix syn pin

[xtqqczze]

Fix the CI failure caused by an ambiguous syn package selection after multiple versions of syn are present in the dependency graph.

The failing command produces:

error: There are multiple `syn` packages in your project, and the specification `syn` is ambiguous.
Please re-run this command with `-p <spec>` where `<spec>` is one of the following:
  syn@2.0.119
  syn@3.0.3