CI: Cache archives downloaded by apt (#8159)

Belt-and-braces in case GitHub apt mirrors start acting up again: On any
successful apt install, cache the deb files downloaded/needed for the
step, so on next run the mirrors need not be consulted. (That is: trade
apt mirror access with GitHub cache access, which is more reliable)

.github/actions/install-deps/action.yml

@@ -0,0 +1,109 @@
+---
+# DESCRIPTION: Github actions composite action
+# SPDX-License-Identifier: LGPL-3.0-only OR Artistic-2.0
+
+name: Install dependencies
+description: >-
+  Install the dependencies of a CI stage with 'ci/ci-install.bash', caching the
+  '.deb' files apt downloads for that stage, so a package is fetched from the
+  network at most once per runner image. On hosts that do not use apt this is
+  just the install, there is nothing to cache.
+
+inputs:
+  repo:
+    description: "Verilator checkout holding the ci/ scripts"
+    required: false
+    default: repo
+  stage:
+    description: "CI stage to install the dependencies of, e.g. 'build'"
+    required: true
+
+runs:
+  using: composite
+  steps:
+    - name: Compute apt cache key
+      id: cachekey
+      if: ${{ runner.os == 'Linux' }}
+      shell: bash
+      working-directory: ${{ inputs.repo }}
+      env:
+        STAGE: ${{ inputs.stage }}
+      run: |
+        source ci/ci-common.bash
+        host="${DISTRO_ID:-$HOST_OS}${DISTRO_VERSION:+-$DISTRO_VERSION}"
+        echo "key=apt-${host}-${STAGE}" >> "$GITHUB_OUTPUT"
+
+    - name: Restore apt downloads
+      if: ${{ runner.os == 'Linux' }}
+      uses: actions/cache/restore@55cc8345863c7cc4c66a329aec7e433d2d1c52a9  # v5
+      with:
+        path: ${{ runner.temp }}/apt-cache
+        key: ${{ steps.cachekey.outputs.key }}
+        restore-keys: |
+          ${{ steps.cachekey.outputs.key }}-
+
+    - name: Seed apt archive from cache
+      if: ${{ runner.os == 'Linux' }}
+      shell: bash
+      env:
+        APT_CACHE_DIR: ${{ runner.temp }}/apt-cache
+      run: |
+        set -euo pipefail
+        # Ask apt where it keeps its downloads, rather than assume
+        eval "$(apt-config shell ARCHIVES Dir::Cache::archives/d)"
+        ARCHIVES=${ARCHIVES%/}  # '/d' yields a trailing slash
+        # Wipe the archive first, so that whatever the runner image left behind is not stored
+        sudo apt-get -o DPkg::Lock::Timeout=60 clean
+        # The directory is absent when nothing was restored, e.g. on a cold start
+        if [ -d "$APT_CACHE_DIR" ] &&
+          [ -n "$(find "$APT_CACHE_DIR" -maxdepth 1 -name '*.deb' -print -quit)" ]; then
+          sudo cp "$APT_CACHE_DIR"/*.deb "$ARCHIVES"
+        fi
+
+    - name: Install dependencies
+      shell: bash
+      working-directory: ${{ inputs.repo }}
+      env:
+        STAGE: ${{ inputs.stage }}
+      run: ./ci/ci-install.bash "$STAGE"
+
+    - name: Collect apt downloads
+      id: collect
+      if: ${{ runner.os == 'Linux' }}
+      shell: bash
+      env:
+        APT_CACHE_DIR: ${{ runner.temp }}/apt-cache
+      run: |
+        set -euo pipefail
+        # Ask apt where it keeps its downloads, rather than assume
+        eval "$(apt-config shell ARCHIVES Dir::Cache::archives/d)"
+        ARCHIVES=${ARCHIVES%/}  # '/d' yields a trailing slash
+        # Sorted names of the '.deb' files in a directory, empty if it has none
+        list-debs() {
+          [ -d "$1" ] || return 0
+          find "$1" -maxdepth 1 -name '*.deb' -printf '%f\n' | sort
+        }
+        # Superseded downloads can no longer be fetched, so drop them
+        sudo apt-get -o DPkg::Lock::Timeout=60 autoclean
+        downloaded=$(list-debs "$ARCHIVES")
+        if [ -z "$downloaded" ]; then
+          echo "Apt kept no '.deb' files, nothing to cache"
+          exit 0
+        fi
+        if [ "$downloaded" = "$(list-debs "$APT_CACHE_DIR")" ]; then
+          echo "Downloads unchanged, keeping the restored cache entry"
+          exit 0
+        fi
+        # Mirror the archive, rather than add to it, so that the cache cannot
+        # grow without bound as package versions churn
+        mkdir -p "$APT_CACHE_DIR"
+        rm -f "$APT_CACHE_DIR"/*.deb
+        sudo cp "$ARCHIVES"/*.deb "$APT_CACHE_DIR"
+        echo "changed=true" >> "$GITHUB_OUTPUT"
+
+    - name: Save apt downloads
+      if: ${{ steps.collect.outputs.changed == 'true' }}
+      uses: actions/cache/save@55cc8345863c7cc4c66a329aec7e433d2d1c52a9  # v5
+      with:
+        path: ${{ runner.temp }}/apt-cache
+        key: ${{ steps.cachekey.outputs.key }}-${{ github.run_id }}