Replaced actions/checkout usage with manual download commands.

This fixes the deprecation warnings for actions/checkout@v3. actions/checkout@v4
is not functional because of the upstream bug:

actions/checkout#1590

.github/workflows/ci.yml

@@ -1,4 +1,4 @@
-# Copyright 2021-2023 Andrey Semashev
+# Copyright 2021-2024 Andrey Semashev
 #
 # Distributed under the Boost Software License, Version 1.0.
 # (See accompanying file LICENSE_1_0.txt or copy at http://boost.org/LICENSE_1_0.txt)
@@ -317,8 +317,6 @@ jobs:
             fi
             git config --global pack.threads 0
 
-      - uses: actions/checkout@v3
-
       - name: Install packages
         if: matrix.install
         run: |
@@ -433,11 +431,25 @@ jobs:
             then
                 DEPINST_ARGS+=("--git_args" "--jobs $GIT_FETCH_JOBS")
             fi
+            mkdir -p snapshot
+            cd snapshot
+            echo "Downloading library snapshot: https://github.com/${GITHUB_REPOSITORY}/archive/${GITHUB_SHA}.tar.gz"
+            curl -L --retry "$NET_RETRY_COUNT" -o "${LIBRARY}-${GITHUB_SHA}.tar.gz" "https://github.com/${GITHUB_REPOSITORY}/archive/${GITHUB_SHA}.tar.gz"
+            tar -xf "${LIBRARY}-${GITHUB_SHA}.tar.gz"
+            if [ ! -d "${LIBRARY}-${GITHUB_SHA}" ]
+            then
+                echo "Library snapshot does not contain the library directory ${LIBRARY}-${GITHUB_SHA}:"
+                ls -la
+                exit 1
+            fi
+            rm -f "${LIBRARY}-${GITHUB_SHA}.tar.gz"
             cd ..
             git clone -b "$BOOST_BRANCH" --depth 1 "https://github.com/boostorg/boost.git" "boost-root"
             cd boost-root
-            mkdir -p libs/$LIBRARY
-            cp -r $GITHUB_WORKSPACE/* libs/$LIBRARY
+            mkdir -p libs
+            rm -rf "libs/$LIBRARY"
+            mv -f "../snapshot/${LIBRARY}-${GITHUB_SHA}" "libs/$LIBRARY"
+            rm -rf "../snapshot"
             git submodule update --init tools/boostdep
             DEPINST_ARGS+=("$LIBRARY")
             python tools/boostdep/depinst/depinst.py "${DEPINST_ARGS[@]}"
@@ -459,7 +471,7 @@ jobs:
       - name: Run tests
         if: matrix.cmake_tests == ''
         run: |
-            cd ../boost-root
+            cd boost-root
             B2_ARGS=("-j" "$BUILD_JOBS" "toolset=${{matrix.toolset}}" "cxxstd=${{matrix.cxxstd}}")
             if [ -n "${{matrix.build_variant}}" ]
             then