fix(ci): build-packages arch=both must expand to both arches (ref #760)

The discover matrix filter only compared requested_arch against amd64/arm64/
empty, so the 'both' workflow_dispatch option produced an EMPTY matrix (no
builds, collect failed). Normalize 'both' -> empty (= all arches).

.github/workflows/build-packages.yml

@@ -63,6 +63,11 @@ jobs:
           # Build the flat {package, arch} matrix. Honour the workflow_dispatch
           # `arch` and `package` filters if set (empty on `push: tags` events).
           requested_arch="${REQUESTED_ARCH:-}"
+          # `both` means build every arch — same as the empty (push: tags)
+          # case. Without this the matrix filter (which only compares against
+          # amd64/arm64/empty) yields an EMPTY matrix, so no package builds and
+          # `collect` fails.
+          [ "$requested_arch" = "both" ] && requested_arch=""
           requested_pkg="${REQUESTED_PKG:-}"
 
           combos=$(find packages/secubox-* -path "*/debian/control" -not -path "*/debian/*/DEBIAN/control" \
@@ -152,7 +157,12 @@ jobs:
           sudo apt-get update -qq
           sudo apt-get install -y -qq \
             build-essential dpkg-dev debhelper devscripts fakeroot \
-            dh-python python3-all python3-setuptools
+            dh-python python3-all python3-setuptools golang-go
+          # golang-go satisfies Build-Depends of the pure-Go packages
+          # (secubox-dpi, secubox-toolbox-ng: CGO_ENABLED=0, GOARCH=arm64,
+          # -mod=vendor offline cross-compile). ubuntu-24.04 ships >= 1.22.
+          # Without it dpkg-checkbuilddeps aborts the arm64 build — this was
+          # the real cause of the "arm64 red" runs, not a CGO toolchain gap.
           # arm64 cross-toolchain — dh_strip and dh_makeshlibs invoke
           # aarch64-linux-gnu-{strip,objdump} when -a arm64 is passed.
           # Without these, arch-specific packages shipping prebuilt