Fix CI: add cmake and its transitive runtime deps to the toolchain bootstrap

yajl (needed by conmon, bootstrapped from source since Slackware ships
no package for it) builds via CMake, not the cmake-free autoconf script
its ./configure wrapper name suggests. Tracing the failure through the
actual vbatts/slackware:15.0 image on the runner host surfaced a chain
of packages slackpkg does not auto-resolve (Slackware packages carry no
dependency metadata at all): cmake needs libarchive, which needs lz4 and
libxml2; the patched make/gmake this mirror serves needs guile, which
needs gc; compiling anything needs kernel-headers for <linux/errno.h>;
and this build's binutils (ar/ranlib) needs flex, while objdump needs
elfutils. All added to the same slackpkg install list as the rest of
the toolchain, keeping everything on one mutually consistent version
set. Verified end-to-end against vbatts/slackware:15.0 on the actual
runner host at each step of this dependency chain.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

scripts/ci/setup-slackware-buildenv.sh

@@ -75,7 +75,8 @@ if command -v slackpkg > /dev/null 2>&1; then
   slackpkg -batch=on -default_answer=y update
   slackpkg -batch=on -default_answer=y install \
     gcc gcc-g++ binutils make m4 perl autoconf automake libtool pkg-config \
-    curl nghttp2 brotli cyrus-sasl ca-certificates glib2 libcap fuse3
+    curl nghttp2 brotli cyrus-sasl ca-certificates glib2 libcap fuse3 \
+    cmake libarchive lz4 libxml2 guile gc kernel-headers flex elfutils
 else
   echo "==> slackpkg not found, assuming toolchain is already provided by the base image"
 fi
@@ -134,7 +135,9 @@ if ! pkg-config --exists yajl 2>/dev/null; then
     exit 1
   }
   mkdir -p "$d/src" && tar -xf "$d/src.tar.gz" -C "$d/src" --strip-components=1
-  # yajl uses its own cmake-free ./configure wrapper script.
+  # yajl's ./configure is a thin wrapper around CMake (not a cmake-free
+  # autoconf script, despite its name) — cmake must already be on PATH,
+  # see the slackpkg install list in step 0 above.
   ( cd "$d/src" && ./configure -p /usr && make -C build install )
   ldconfig 2>/dev/null || true
 else