ci(pi-gen): native arm64 runner skips QEMU entirely

usimd/pi-gen-action#179: Trixie + QEMU breaks on x86 runners (arch-test
"arm64: not supported"). Native arm64 runner means no qemu, no binfmt
registration dance — pi-gen runs the chroot directly. Faster too.

.github/workflows/build.yml

@@ -125,10 +125,13 @@ jobs:
           retention-days: 14
 
   # ---- Flashable Pi OS Trixie image (aarch64 only) -------------------------
+  # Native arm64 runner avoids QEMU entirely — pi-gen runs faster + sidesteps
+  # the Trixie binfmt regression (usimd/pi-gen-action#179). With native arm64
+  # the chroot's binaries match the kernel, so arch-test passes immediately.
   image:
     if: ${{ inputs.build-image }}
     needs: binary
-    runs-on: ubuntu-latest
+    runs-on: blacksmith-4vcpu-ubuntu-2404-arm
     steps:
       - uses: actions/checkout@v6
         with:
@@ -168,32 +171,6 @@ jobs:
              deploy/pi-gen/stage-betterframe-client/01-install-kiosk/files/
           chmod +x deploy/pi-gen/stage-betterframe-client/01-install-kiosk/00-run-chroot.sh
 
-      # x86 runner can't natively execute the arm64 binaries pi-gen drops
-      # into the chroot. tonistiigi/binfmt registers QEMU with the F flag
-      # (kernel preloads the static binary), making it visible inside
-      # pi-gen's nested container kernel-namespace-share. This is what
-      # docker/setup-qemu-action wraps, called directly here so we control
-      # the flags + can sanity-check after.
-      - name: Register QEMU binfmt for arm64
-        # Host-install qemu-user-static (Debian/Ubuntu package). It auto-
-        # registers binfmt_misc entries pointing at
-        # /usr/bin/qemu-aarch64-static. The Debian post-install also sets
-        # the F (fix-binary) flag so the kernel preloads the static binary
-        # bytes — meaning the chroot inside pi-gen doesn't need a copy.
-        # tonistiigi/binfmt only ships /usr/bin/qemu-aarch64 (dynamic),
-        # which fails inside a chroot at exec time when ld.so + libs are
-        # missing. Pi-gen specifically wants the -static path.
-        run: |
-          sudo apt-get update
-          sudo apt-get install -y --no-install-recommends \
-            qemu-user-static binfmt-support
-          sudo update-binfmts --enable qemu-aarch64
-          echo "--- binfmt_misc registrations ---"
-          ls -la /proc/sys/fs/binfmt_misc/ || true
-          echo "--- qemu-aarch64 detail (interpreter should be -static) ---"
-          cat /proc/sys/fs/binfmt_misc/qemu-aarch64 || true
-          file /usr/bin/qemu-aarch64-static || true
-
       - name: Build Pi image (pi-gen)
         uses: usimd/pi-gen-action@v1
         with: