[CI] Stop persisting Linux caches instead of caching the image

Measuring the follow-up showed the image half costs more than it frees:
the nine build images run 1.1 to 2.4 GB each on disk, about 5 to 6 GB of
cache quota for all of them once compressed, against the roughly 2.6 GB
the Linux ccache entries occupy. Persisting the image would push the
repository further over quota instead of under it, so these legs now
persist nothing: the image is rebuilt every run as before, and ccache
stays a throwaway dir shared only between the steps of one run.

.github/workflows/desktop-build.yml

@@ -168,7 +168,7 @@ jobs:
     continue-on-error: ${{ matrix.allow-failure == 'yes' }}
     timeout-minutes: 70
     env:
-      CACHE: ${{ github.workspace }}/.cache/${{ matrix.distro }}${{ matrix.version }}${{ matrix.variant }}    # directory holding the build image and ccache, only the image subdir is cached below
+      CACHE: ${{ github.workspace }}/.cache/${{ matrix.distro }}${{ matrix.version }}${{ matrix.variant }}    # scratch dir for the container image build and throwaway ccache; nothing below is cached
       CCACHE_EVICTION_AGE: 7d
       CCACHE_SIZE: 600M    # space of all repo is 10Gi: https://docs.github.com/en/actions/using-workflows/caching-dependencies-to-speed-up-workflows#usage-limits-and-eviction-policy
       CMAKE_GENERATOR: 'Ninja'
@@ -179,29 +179,16 @@ jobs:
       - name: "Checkout"
         uses: actions/checkout@v7
 
-      # Only the image half of this cache is worth keeping. A compile on these legs runs
-      # inside a container, so ccache measures a 0.00 to 1.85 percent hit rate here against
-      # 49 to 50 percent on the macOS legs, and the roughly 420 MiB of ccache each entry
-      # carried was buying nothing. Caching $CACHE/image alone drops that.
-      # The key changes name because the cached content does: the old ccache-<distro>
-      # entries hold an image plus a ccache directory, and expire on their own untouched.
-      - name: "Restore build image cache"
-        id: restore_image_cache
-        uses: actions/cache/restore@v6
-        with:
-          key: image-${{ matrix.distro }}${{ matrix.version }}${{ matrix.variant }}-${{ hashFiles(format('.ci/{0}{1}/**', matrix.distro, matrix.version)) }}
-          path: ${{ env.CACHE }}/image
-
+      # Nothing on these legs is worth persisting. A compile here runs inside a
+      # container, so ccache measures a 0.00 to 1.85 percent hit rate against 49 to
+      # 50 percent on the macOS legs: persisting it spent a few hundred MiB per
+      # entry buying nothing. The build image is not an alternative: it runs 1.1 to
+      # 2.4 GB per leg on disk, about 5 to 6 GB of quota for all nine legs once
+      # compressed, so caching it would cost more than dropping the ccache frees.
+      # The old ccache-<distro> entries are never read again and expire on their own.
       - name: "Build ${{ matrix.distro }} ${{ matrix.version }} Docker image"
         shell: bash
-        run: |
-          # --get/--save wire the $CACHE/image dir restored/saved above into the
-          # build: without them the image is rebuilt from scratch every run and
-          # the image cache saves nothing. Only persist on master; PR runs just
-          # reuse whatever the restore step found.
-          args=(--get --build)
-          [[ $GITHUB_REF == "refs/heads/master" ]] && args+=(--save)
-          source .ci/docker.sh "${args[@]}"
+        run: source .ci/docker.sh --build
 
       - name: "Build debug and test"
         if: matrix.test != 'skip'
@@ -263,25 +250,6 @@ jobs:
 
           RUN --server --release --package "$package" "${args[@]}"
 
-      # Delete used cache to emulate a cache update. See https://github.com/actions/cache/issues/342
-      - name: "[Linux] Delete used cache to emulate a cache update"
-        if: github.ref == 'refs/heads/master' && steps.restore_image_cache.outputs.cache-hit
-        continue-on-error: true
-        env:
-          CACHE_PRIMARY_KEY: ${{ steps.restore_image_cache.outputs.cache-primary-key }}
-          GH_TOKEN: ${{ github.token }}
-        run: |
-          if gh cache delete --repo "$GITHUB_REPOSITORY" "$CACHE_PRIMARY_KEY"; then
-            echo "Cache deleted successfully"
-          fi
-
-      - name: "Save updated build image cache"
-        if: github.ref == 'refs/heads/master'
-        uses: actions/cache/save@v6
-        with:
-          key: ${{ steps.restore_image_cache.outputs.cache-primary-key }}
-          path: ${{ env.CACHE }}/image
-
       - name: "Upload artifact"
         id: upload_artifact
         if: matrix.package != 'skip'