set serde_json to 1.0.100 in CI s.t. we can build with Rust 1.41.1

Author: sander2

.github/workflows/rust.yml

@@ -27,6 +27,7 @@ jobs:
                   profile: minimal
                   toolchain: ${{ matrix.rust }}
                   override: true
+            - run: cargo update -p serde_json --precise 1.0.100
             - run: cargo update -p serde --precise 1.0.189
             - name: Running test script
               env: ${{ matrix.env }}