Switch to Debian apt packages, pin nrfx to v2.9.0

- Replace ARM tarball download with gcc-arm-none-eabi from apt
- Drop TARGETARCH/GCC_VERSION build args — no longer needed
- Pin nrfx submodule to v2.9.0 which still bundles mdk/ headers
- nrfx 3+ dropped mdk/ from the repo

justfile

@@ -13,17 +13,11 @@ user_ns := `command -v podman >/dev/null 2>&1 \
             && echo "--userns=keep-id" \
             || echo "--user $(id -u):$(id -g)"`
 
-# host CPU arch → container build arg
-arch := `uname -m | sed 's/x86_64/amd64/; s/aarch64/arm64/'`
-
 # ── recipes ────────────────────────────────────────────────────────────────
 
 # build container image (only rebuilt when Dockerfile changes)
 image-build:
-    {{engine}} build \
-        --build-arg TARGETARCH={{arch}} \
-        --build-arg GCC_VERSION=13.2.rel1 \
-        -t {{image}} .
+    {{engine}} build -t {{image}} .
 
 # compile firmware inside the container
 build: image-build