ci: cache Bitcoin Core source builds with ccache

Add ccache to the Bitcoin Core source build job using the same basic pattern as bitcoin-capnp-types: install ccache, restore ~/.cache/ccache, and use ccache as the CMake compiler launcher.

Behavior changes:
- Bitcoin Core source build jobs now restore and save a per-OS ccache cache.
- Bitcoin Core C and C++ compilation goes through ccache.
- The cache step uses actions/cache@v6.1.0, which runs on Node 24.
- Integration-test artifact upload/download steps use current Node 24-compatible artifact actions.

Author: Sjors

.github/workflows/integration-tests.yaml

@@ -52,7 +52,7 @@ jobs:
           cargo nextest archive --manifest-path=integration-tests/Cargo.toml --archive-file integration-test-binaries-${{ runner.os }}.tar.zst
 
       - name: Upload compiled test archive
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v7
         with:
           name: integration-test-binaries-${{ runner.os }}
           path: integration-test-binaries-${{ runner.os }}.tar.zst
@@ -83,11 +83,18 @@ jobs:
 
       - name: Install Bitcoin Core build dependencies (Ubuntu)
         if: matrix.os == 'ubuntu-latest'
-        run: sudo apt-get install -y build-essential cmake pkgconf python3 libevent-dev libboost-dev libsqlite3-dev capnproto libcapnp-dev
+        run: sudo apt-get install -y build-essential cmake pkgconf python3 libevent-dev libboost-dev libsqlite3-dev capnproto libcapnp-dev ccache
 
       - name: Install Bitcoin Core build dependencies (macOS)
         if: matrix.os == 'macos-latest'
-        run: brew install cmake boost pkgconf libevent sqlite capnp
+        run: brew install cmake boost pkgconf libevent sqlite capnp ccache
+
+      - name: ccache
+        uses: actions/cache@v6.1.0
+        with:
+          path: ~/.cache/ccache
+          key: ccache-bitcoin-${{ runner.os }}-${{ github.sha }}
+          restore-keys: ccache-bitcoin-${{ runner.os }}-
 
       - name: Checkout Bitcoin Core source
         run: |
@@ -98,11 +105,12 @@ jobs:
       - name: Build Bitcoin Core
         run: |
           cd bitcoin
-          cmake -B build -DENABLE_WALLET=ON -DBUILD_TESTS=OFF
+          export CCACHE_DIR="$HOME/.cache/ccache"
+          cmake -B build -DENABLE_WALLET=ON -DBUILD_TESTS=OFF -DCMAKE_C_COMPILER_LAUNCHER=ccache -DCMAKE_CXX_COMPILER_LAUNCHER=ccache
           cmake --build build -j "$(nproc 2>/dev/null || sysctl -n hw.ncpu)"
 
       - name: Upload Bitcoin Core binary
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v7
         with:
           name: bitcoin-core-${{ runner.os }}-${{ steps.bitcoin-core.outputs.cache-key }}
           path: bitcoin/build/bin/bitcoin-node
@@ -204,7 +212,7 @@ jobs:
 
       - name: Download Bitcoin Core binary
         if: steps.bitcoin-core.outputs.source == 'true'
-        uses: actions/download-artifact@v4
+        uses: actions/download-artifact@v8
         with:
           name: bitcoin-core-${{ runner.os }}-${{ steps.bitcoin-core.outputs.cache-key }}
           path: bitcoin-core
@@ -219,7 +227,7 @@ jobs:
           github-token: ${{ github.token }}
 
       - name: Download compiled test archive
-        uses: actions/download-artifact@v4
+        uses: actions/download-artifact@v8
         with:
           name: integration-test-binaries-${{ runner.os }}