chore: fix msrv check by pinning syn (#519)

Author: mbrobbel

.github/workflows/rust.yml

@@ -64,6 +64,7 @@ jobs:
           cargo update -p parking_lot --precise 0.12.4
           cargo update -p parking_lot_core --precise 0.9.11
           cargo update -p lock_api --precise 0.4.13
+          cargo update -p syn --precise 2.0.106
       - name: Check
         run: |
           # run `cargo msrv verify` to see problems