CI: use cargo fuzz 0.8.X

It has switched to release+debug assertion builds by default, so pass `--dev` to
avoid compiling with optimizations.

.github/workflows/main.yml

@@ -124,10 +124,10 @@ jobs:
     - uses: ./.github/actions/install-rust
       with:
         toolchain: nightly-2020-06-03
-    - run: cargo install cargo-fuzz --vers "^0.7"
+    - run: cargo install cargo-fuzz --vers "^0.8"
     - run: cargo fetch
       working-directory: ./fuzz
-    - run: cargo fuzz build --features binaryen
+    - run: cargo fuzz build --dev --features binaryen
 
   rebuild_peephole_optimizers:
     name: Rebuild Peephole Optimizers