Cache cargo-fuzz between invocations

This will make multiple runs on the same day faster since they can
reuse the cache. Fuzzing uses nightly Rust, so the cache will expire
daily.

.github/workflows/build.yml

@@ -103,6 +103,16 @@ jobs:
       - name: Install nightly Rust
         run: rustup default nightly
 
+      - name: Get nightly Rust version
+        id: rustc-version
+        run: echo "::set-output name=version::$(rustc --version)"
+
+      - name: Cache cargo-fuzz
+        uses: actions/cache@v2
+        with:
+          path: ~/.cargo/bin/cargo-fuzz
+          key: cargo-fuzz-${{ steps.rustc-version.outputs.version }}
+
       - name: Install cargo-fuzz
         run: cargo install cargo-fuzz