Use older cbindgen on android to work around MSRV

.github/workflows/build.yml

@@ -661,7 +661,11 @@ jobs:
       - name: Install cbindgen
         run: |
           . $HOME/.cargo/env
-          cargo install cbindgen
+          git clone https://github.com/eqrion/cbindgen
+          cd cbindgen/
+          git checkout v0.20.0
+          cargo update -p indexmap --precise "1.6.2" --verbose
+          cargo install --locked --path .
       - name: Checkout Rust-Lightning and LDK-C-Bindings git
         run: |
           git config --global user.email "ldk-ci@example.com"