ci(build): persist cargo registry via CARGO_HOME on existing volume

The compose mount `./runner-cargo-target:/cache/target` already
persists the target dir. Set CARGO_HOME to a subdir of that mount so
the downloaded-crates index + git cache survive too — otherwise every
build re-fetches every dep. No server-side compose change needed.

.gitea/workflows/build-and-publish.yaml

@@ -16,6 +16,13 @@ jobs:
     steps:
       - name: Build and publish changed packages
         run: |
+          # Persist cargo registry + git cache across runs by piggy-backing
+          # on the existing persistent CARGO_TARGET_DIR volume (compose
+          # mounts ./runner-cargo-target -> /cache/target). Without this,
+          # every build re-downloads every crate.
+          export CARGO_HOME="${CARGO_TARGET_DIR:-/cache/target}/cargo-home"
+          mkdir -p "$CARGO_HOME"
+
           rm -rf repo
           git clone http://gitea:3000/nevaforget/moonarch-pkgbuilds.git repo
           cd repo