ci: drop the build cache from tag image builds

GitHub scopes a run's Actions cache to its ref, so the cache a tag build
wrote could only ever be read by that same tag: the next release built cold
anyway. Each release also parked several GB of Rust layers in the
repository's 10 GB cache, enough to evict main's cargo cache and slow
everyday builds too. The image builds now run without a cache.

.github/workflows/ci.yml

@@ -236,17 +236,16 @@ jobs:
           username: jcoffey-dev
           password: ${{ secrets.GITEA_TOKEN }}
       # Attestations off: they add manifests of their own, and the index
-      # should hold the two images and nothing else. The GitHub Actions cache
-      # keeps the dependency layer (`cargo chef cook`), which only a
-      # dependency change alters, between releases.
+      # should hold the two images and nothing else. No build cache: GitHub
+      # scopes a tag run's cache to that tag, so the next release could never
+      # read it, and each one would park several GB in the repository's 10 GB
+      # cache and evict main's cargo cache.
       - uses: docker/build-push-action@c3c9e263c25d99ce0380d002d59b67737d91b0dc # v7.4.0
         with:
           context: .
           platforms: linux/${{ matrix.arch }}
           provenance: false
           sbom: false
-          cache-from: type=gha,scope=image-${{ matrix.arch }}
-          cache-to: type=gha,mode=max,scope=image-${{ matrix.arch }}
           push: true
           tags: |
             ${{ env.IMAGE }}:${{ env.VERSION }}-${{ matrix.arch }}