Pin last working syn version in msrv-check

See dtolnay/syn@92a0aa4

A first version if this failed with the following, so the exact version to
update is specified:

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@1.0.109
  syn@2.0.117

Signed-off-by: Uli Schlachter <psychon@znc.in>

Author: psychon

.github/workflows/CI.yml

@@ -189,6 +189,8 @@ jobs:
       run: cargo update --package async-lock --precise 3.4.1
     - name: Pin last unicode-ident release supporting our msrv of Rust 1.68
       run: cargo update --package unicode-ident --precise 1.0.22
+    - name: Pin last syn release supporting our msrv of Rust 1.68
+      run: cargo update --package syn@2.0.117 --precise 2.0.114
 
     # build
     - name: cargo check x11rb-protocol with all features