ci: update rustc version for fuzz job

This was at 1.58 to handle honggfuzz breakage; apparently it has to be
1.65 now.

Author: apoelstra

.github/workflows/fuzz.yml

@@ -39,7 +39,7 @@ roundtrip_semantic,
           key: cache-${{ matrix.target }}-${{ hashFiles('**/Cargo.toml','**/Cargo.lock') }}
       - uses: actions-rs/toolchain@v1
         with:
-          toolchain: 1.58
+          toolchain: 1.65
           override: true
           profile: minimal
       - name: fuzz