Cache packages in Linux CI

Compilers are installed from third-party archives that periodically
return 50x or throttle to a crawl, which fails the build. Keep the
downloaded packages and restore them on the next run so that both apt
and the legacy wget-based installs work from disk instead of refetching.

apt, unlike apt-get, discards downloaded packages after installing them
unless the binary-specific Keep-Downloaded-Packages is set.

The cache is written once per key; bump the -v1 suffix to refresh it
after the package versions change.

.github/workflows/linux.yml

@@ -86,13 +86,27 @@ jobs:
     steps:
     - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
 
+    # Packages are installed from third-party archives that periodically
+    # return 50x or throttle, which otherwise fails the build. The key covers
+    # everything that determines the package set; bump the -v1 suffix to
+    # discard the cached packages.
+    - name: Cache packages
+      uses: actions/cache@55cc8345863c7cc4c66a329aec7e433d2d1c52a9 # v6.1.0
+      with:
+        path: ~/apt-cache
+        key: pkg-${{matrix.os || 'ubuntu-22.04'}}-${{matrix.cxx}}-${{matrix.install}}-v1
+
+    - name: Create package cache directory
+      run: mkdir -p ~/apt-cache
+
     - name: Install GCC 4.9
       run: |
         sudo apt update
         sudo apt install libatomic1 libc6-dev libgomp1 libitm1 libmpc3
+        cd ~/apt-cache
         # https://launchpad.net/ubuntu/xenial/amd64/g++-4.9/4.9.3-13ubuntu2
         # launchpad periodically returns 50x errors, so retry to avoid flaky CI.
-        wget --no-verbose --tries=5 --waitretry=10 \
+        wget --no-verbose --no-clobber --tries=5 --waitretry=10 \
           --retry-connrefused --retry-on-http-error=502,503,504 \
           http://launchpadlibrarian.net/230069137/libmpfr4_3.1.3-2_amd64.deb \
           http://launchpadlibrarian.net/253728424/libasan1_4.9.3-13ubuntu2_amd64.deb \
@@ -123,9 +137,10 @@ jobs:
       run: |
         sudo apt update
         sudo apt install libtinfo5
+        cd ~/apt-cache
         # https://code.launchpad.net/ubuntu/xenial/amd64/clang-3.6/1:3.6.2-3ubuntu2
         # launchpad periodically returns 50x errors, so retry to avoid flaky CI.
-        wget --no-verbose --tries=5 --waitretry=10 \
+        wget --no-verbose --no-clobber --tries=5 --waitretry=10 \
           --retry-connrefused --retry-on-http-error=502,503,504 \
           http://launchpadlibrarian.net/230019046/libffi6_3.2.1-4_amd64.deb \
           http://launchpadlibrarian.net/445346109/libasan2_5.4.0-6ubuntu1~16.04.12_amd64.deb \
@@ -186,9 +201,15 @@ jobs:
 
     - name: Create build environment
       run: |
+        # apt (unlike apt-get) defaults to discarding downloaded packages, so
+        # the binary-specific setting is required to populate the cache.
+        printf 'Binary::apt::APT::Keep-Downloaded-Packages "true";\n' | \
+          sudo tee /etc/apt/apt.conf.d/99keep-packages
+        sudo cp -n ~/apt-cache/*.deb /var/cache/apt/archives || true
         sudo apt update
         ${{matrix.install}}
         sudo apt install locales-all
+        cp -n /var/cache/apt/archives/*.deb ~/apt-cache || true
         cmake -E make_directory ${{runner.workspace}}/build
 
     - name: Select matching C compiler