Update Makefile target in release workflow

There's another location which uses the "manifests directory" target
directly, but isn't run when testing a PR: the release workflow.

Signed-off-by: Michael Bridgen <michael@weave.works>

.github/workflows/release.yaml

@@ -50,7 +50,7 @@ jobs:
         uses: fluxcd/pkg//actions/kustomize@main
       - name: Generate manifests
         run: |
-          make cmd/flux/manifests
+          make cmd/flux/.manifests.done
           ./manifests/scripts/bundle.sh "" ./output manifests.tar.gz
           kustomize build ./manifests/install > ./output/install.yaml
       - name: Build CRDs