Pin quote and proc-macro2 as required to get CI passing MSRV

Author: TheBlueMatt

.github/workflows/build.yml

@@ -29,6 +29,11 @@ jobs:
         uses: actions/checkout@v2
         with:
           fetch-depth: 0
+      - name: Pin syn and quote to meet MSRV
+        run: |
+          cd lightning-c-bindings
+          cargo update -p syn --precise "2.0.106" --verbose
+          cargo update -p quote --precise "1.0.41" --verbose
       - name: Sanity test bindings against Cargo.toml RL
         working-directory: lightning-c-bindings
         run: |