ci: split fuzz sanity check into separate parallel job

The sanity check (cargo test on fuzz targets) doesn't use the
restored corpus and was blocking the actual fuzz run. Move it to
a separate fuzz_sanity job so both run in parallel.

AI tools were used in preparing this commit.

Author: joostjager

.github/workflows/build.yml

@@ -205,6 +205,21 @@ jobs:
       - name: Simulate docs.rs build
         run: ci/check-docsrs.sh
 
+  fuzz_sanity:
+    runs-on: self-hosted
+    env:
+      TOOLCHAIN: 1.75
+    steps:
+      - name: Checkout source code
+        uses: actions/checkout@v4
+      - name: Install Rust ${{ env.TOOLCHAIN }} toolchain
+        run: |
+          curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --profile=minimal --default-toolchain ${{ env.TOOLCHAIN }}
+      - name: Sanity check fuzz targets on Rust ${{ env.TOOLCHAIN }}
+        run: |
+          cd fuzz
+          RUSTFLAGS="--cfg=fuzzing --cfg=secp256k1_fuzz --cfg=hashes_fuzz" cargo test --quiet --color always --lib --bins -j8
+
   fuzz:
     runs-on: self-hosted
     env:
@@ -238,11 +253,6 @@ jobs:
           key: fuzz-corpus-refs/heads/main-${{ github.sha }}
           restore-keys: |
             fuzz-corpus-refs/heads/main-
-      - 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 --lib --bins -j8
-          cargo clean
       - name: Run fuzzers
         run: cd fuzz && ./ci-fuzz.sh && cd ..
       - name: Upload honggfuzz corpus
@@ -308,7 +318,7 @@ jobs:
           TOR_PROXY="127.0.0.1:9050" RUSTFLAGS="--cfg=tor" cargo test --verbose --color always -p lightning-net-tokio
 
   notify-failure:
-    needs: [build-workspace, build-features, build-bindings, build-nostd, build-cfg-flags, build-sync, fuzz, linting, rustfmt, check_release, check_docs, benchmark, ext-test, tor-connect, coverage]
+    needs: [build-workspace, build-features, build-bindings, build-nostd, build-cfg-flags, build-sync, fuzz_sanity, fuzz, linting, rustfmt, check_release, check_docs, benchmark, ext-test, tor-connect, coverage]
     if: failure() && github.ref == 'refs/heads/main'
     runs-on: ubuntu-latest
     permissions: