Builders emulate aarch64 with QEMU 10.2.3 instead of the abandoned 7.2 image

multiarch/qemu-user-static stopped at QEMU 7.2 (January 2023). Under it,
qmake's compiler probe (`g++ -E -v` in toolchain.prf) returns nothing on the
current gcc 16 toolchain, so every qmake package fails on aarch64 with
"failed to parse default include paths from compiler output" (hype, PR #517).
The same PKGBUILD builds under QEMU 10.2.3 and 11.1.

Register through tonistiigi/binfmt at a pinned tag, on both the ephemeral
builder droplets and the rootful-Docker path of setup_qemu, uninstalling any
existing entry first because the tool keeps an older registration in place.
The tag is now the one place that decides what every emulated build runs
under. Flags stay F and C, which rootless sudo inside the builder needs.

helpers/docker-helpers.sh

@@ -91,8 +91,19 @@ setup_qemu() {
     exit 1
   fi
 
-  # Register emulators for builds whose target differs from the host.
-  if ! "$CONTAINER_ENGINE" run --rm --privileged docker.io/multiarch/qemu-user-static --reset -p yes --credential yes >/dev/null 2>&1; then
+  # Register emulators for builds whose target differs from the host, with
+  # the F and C flags (tonistiigi/binfmt always sets both). The image tag pins
+  # the QEMU version every emulated build runs under; multiarch/qemu-user-static
+  # stopped at QEMU 7.2, which breaks qmake's compiler probe on current gcc.
+  # Keep ci/runner-cloud-init.yaml on the same tag. Uninstall first: install
+  # leaves an existing registration (an older emulator) in place and exits 0.
+  local platform_arch
+  case "$target_arch" in
+    aarch64) platform_arch=arm64 ;;
+    x86_64) platform_arch=amd64 ;;
+    *) platform_arch="$target_arch" ;;
+  esac
+  if ! "$CONTAINER_ENGINE" run --rm --privileged docker.io/tonistiigi/binfmt:qemu-v10.2.3-68 --uninstall "qemu-$target_arch" --install "$platform_arch" >/dev/null 2>&1; then
     print_error "Failed to set up QEMU emulation"
     exit 1
   fi