fix(ci): tolerate whitespace in buildx metadata-file digest grep

Same fix as nxtgauge-frontend-solid: buildx writes its metadata file
pretty-printed, so the compact ":"-only grep pattern never matched,
failing the build-and-push step right after a successful push and
skipping the GitOps-release step. Match backend-rust's
whitespace-tolerant pattern.

.forgejo/workflows/build.yaml

@@ -75,7 +75,7 @@ jobs:
             -t "$image_ref" \
             .
 
-          digest="$(grep -o '"containerimage.digest":"sha256:[^"]*"' "$metadata_file" | cut -d'"' -f4)"
+          digest="$(grep -o '"containerimage\.digest"[[:space:]]*:[[:space:]]*"sha256:[^"]*"' "$metadata_file" | grep -o 'sha256:[^"]*')"
           test -n "$digest"
           printf '%s@%s\n' "$REGISTRY_HOST/$REGISTRY_NAMESPACE/nxtgauge-admin-solid" "$digest" > /tmp/admin-solid-image-ref.txt