Clean up fuzz test build to fix disk space usage fuzz CI failures

Author: TheBlueMatt

.github/workflows/build.yml

@@ -229,7 +229,10 @@ jobs:
         run: |
           cd fuzz && cargo update -p regex --precise "1.9.6" --verbose && cd ..
       - name: Sanity check fuzz targets on Rust ${{ env.TOOLCHAIN }}
-        run: cd fuzz && RUSTFLAGS="--cfg=fuzzing --cfg=secp256k1_fuzz --cfg=hashes_fuzz" cargo test --verbose --color always
+        run: |
+          cd fuzz
+          RUSTFLAGS="--cfg=fuzzing --cfg=secp256k1_fuzz --cfg=hashes_fuzz" cargo test --verbose --color always
+          cargo clean
       - name: Run fuzzers
         run: cd fuzz && ./ci-fuzz.sh && cd ..