fix(ci): install xz-utils so Node tarball extraction works

The direct-tarball Node install (switched from NodeSource apt in the last
CI fix) failed with "xz: Cannot exec: No such file or directory" because
Ubuntu 24.04 base doesn't include xz-utils. Node ships .tar.xz by default,
and `tar -xJ` shells out to xz, which was missing.

Add xz-utils to the base apt install alongside git/curl/unzip/etc.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.github/docker/Dockerfile.ci

@@ -23,7 +23,7 @@ RUN sed -i \
 # System deps (retry apt-get update — even Hetzner can blip occasionally)
 RUN for i in 1 2 3; do apt-get update && break || sleep 5; done \
     && apt-get install -y --no-install-recommends \
-    git curl unzip ca-certificates jq bc gpg \
+    git curl unzip xz-utils ca-certificates jq bc gpg \
     && rm -rf /var/lib/apt/lists/*
 
 # GitHub CLI