GHA: Cache boost checkout

Instead of cloning the Boost main repository and later potentially many
submodules (by `depinst.py`) we can cache the last checkout.
On any changes the checkout-and-pull should update it to the appropriate
version.

.github/workflows/ci.yml

@@ -225,6 +225,13 @@ jobs:
           BDDE_ARCH: ${{matrix.arch}}
         run: ci/github/setup_bdde.sh
 
+      - name: Cache Boost checkout
+        uses: actions/cache@v4
+        with:
+          path: boost-root
+          key: boost-checkout-cache-${{matrix.os}}-${{matrix.container}}-${{github.sha}}
+          restore-keys: boost-checkout-cache-${{matrix.os}}-${{matrix.container}}-
+
       - name: Setup Boost
         env:
           B2_ADDRESS_MODEL: ${{matrix.address-model}}
@@ -272,6 +279,10 @@ jobs:
           COVERITY_SCAN_NOTIFICATION_EMAIL: ${{ secrets.COVERITY_SCAN_NOTIFICATION_EMAIL }}
           COVERITY_SCAN_TOKEN: ${{ secrets.COVERITY_SCAN_TOKEN }}
 
+      - name: Prepare for cache
+        if: ${{ env.BOOST_ROOT != '' }}
+        run: rm -rf boost-root; mv "${BOOST_ROOT}" boost-root
+        working-directory: ${{github.workspace}}
   windows:
     defaults:
       run:

ci/common_install.sh

@@ -43,6 +43,8 @@ else
     export BOOST_BRANCH="develop"
 fi
 
+# CI cache might have restored the boost-root folder in the current directory
+[ ! -d boost-root ] || mv boost-root ..
 cd ..
 
 if [ ! -d boost-root ]; then