Fix minimal Rust version in CI

The minimum rust version was bumped, so we need
to install the appropriate version in CI.

Author: jschwe

.github/workflows/cbindgen.yml

@@ -31,7 +31,7 @@ jobs:
 
     - name: Install minimum supported Rust version
       id: msrv
-      uses: dtolnay/rust-toolchain@1.57
+      uses: dtolnay/rust-toolchain@1.64
 
     - name: Build with minimum supported Rust version
       run: |