Avoid caching build artefacts

When we copy the whole boost-root AFTER the build(s) it will contain
also build artefacts such as the bin.v2 folder.
When restoring that later B2 might skip parts of the build although it
should not.
Make a copy after the initial setup & bootstrap.

.github/workflows/ci.yml

@@ -308,8 +308,8 @@ jobs:
           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
+        if: ${{ env.BOOST_ROOT_CACHE != '' }}
+        run: rm -rf boost-root; mv "${BOOST_ROOT_CACHE}" boost-root
         working-directory: ${{github.workspace}}
   windows:
     defaults:

ci/github/install.sh

@@ -33,6 +33,11 @@ fi
 
 . $(dirname "${BASH_SOURCE[0]}")/../common_install.sh
 
+# Save the state before building anything to avoid caching build artefacts
+BOOST_ROOT_CACHE="${BOOST_ROOT}-for-cache"
+echo "BOOST_ROOT_CACHE=$BOOST_ROOT_CACHE" >> $GITHUB_ENV
+cp -r "$BOOST_ROOT" "$BOOST_ROOT_CACHE"
+
 # Persist the environment for all future steps
 
 # Set by common_install.sh