Fix CI: bootstrap the full Slackware build toolchain via slackpkg

The build-packages.yml run was failing for two compounding reasons,
found by testing directly against vbatts/slackware:15.0 on the actual
runner host:

1. The image ships neither git nor its HTTPS runtime libs, so
   actions/checkout failed immediately.
2. It's a minimal rootfs with none of the 'D' (development) series —
   no gcc, make, autoconf, pkg-config, curl, glib2, libcap, or fuse3 —
   contrary to setup-slackware-buildenv.sh's assumption that a "full"
   Slackware install already provides these.

An earlier fix attempt hand-pinned git + its deps (nghttp2, brotli,
cyrus-sasl) by exact file + SHA256 from the base 15.0 release
directory. That drifted out of sync with the newer, patched curl
slackpkg installs later in the same container — same shared library,
two different builds, causing a runtime symbol lookup error. Both
steps now resolve every package through slackpkg's own prioritized
mirror instead, keeping the whole toolchain on one mutually consistent
version set. Verified end-to-end (git ls-remote and curl both succeed
over HTTPS against the real Gitea instance, full toolchain present)
in a fresh vbatts/slackware:15.0 container.

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

scripts/ci/setup-slackware-buildenv.sh

@@ -5,14 +5,22 @@
 # Prepares a Slackware container (see .github/workflows/build-packages.yml)
 # to build all seven packages under packages/. Idempotent and safe to re-run.
 #
-# Strategy: detect what's already present (a stock "full" Slackware 15.0
-# install already provides gcc/make/autotools/glib2/libcap/fuse3) and only
-# bootstrap what's genuinely missing (libseccomp, yajl — neither ships in
-# stock Slackware — plus the Go and Rust toolchains, which no Slackware
-# install ships). This makes the script tolerant of small differences
-# between Slackware base image variants instead of assuming one exact image
-# layout, while still failing loudly if something we cannot self-provision
-# (a C compiler, basically) is missing.
+# Strategy: vbatts/slackware:15.0 (the image build-packages.yml runs this
+# in) is a minimal rootfs — it ships none of the 'D' (development) series,
+# nor glib2/libcap/fuse3/curl. Step 0 below uses slackpkg (already present
+# and pre-configured with a mirror in that image) to install the toolchain
+# packages by name. Slackware packages carry no dependency metadata at all
+# (unlike apt/dnf), so slackpkg does NOT resolve dependencies — the list
+# below must name every package explicitly, including curl's HTTPS
+# runtime libs (nghttp2, brotli, cyrus-sasl), or you get a shared-library
+# error at the first invocation, not an install-time failure. What's left
+# after this (libseccomp, yajl — neither ships in stock Slackware — plus
+# the Go and Rust toolchains, which no Slackware install ships) is
+# bootstrapped from source, further down. This makes the script tolerant
+# of small differences between Slackware base image variants (it skips
+# anything slackpkg reports as already installed) instead of assuming one
+# exact image layout, while still failing loudly if something we cannot
+# self-provision is missing.
 #
 # Exits non-zero with a clear message if a required tool cannot be found or
 # provisioned — this script is meant to run early in CI so failures surface
@@ -49,6 +57,29 @@ require_pkgconfig() {
   return 0
 }
 
+# -----------------------------------------------------------------------------
+# 0. Bootstrap the Slackware toolchain packages via slackpkg, if missing.
+#
+# CHECKGPG is turned off here: slackpkg's default GPG-key bootstrap fetches
+# Slackware's signing key from www.slackware.com, which is not reachable
+# from every CI network (observed to hang/fail on the self-hosted Gitea
+# Actions runner this project builds on). slackpkg's CHECKMD5 (on by
+# default) still verifies every package against the mirror's own
+# CHECKSUMS.md5 as a transit-integrity check. This is build-toolchain
+# provisioning, not the shipped artifacts — those are independently
+# checksummed by scripts/checksums.sh.
+# -----------------------------------------------------------------------------
+if command -v slackpkg > /dev/null 2>&1; then
+  echo "==> bootstrapping build toolchain via slackpkg"
+  sed -i 's/^CHECKGPG=on/CHECKGPG=off/' /etc/slackpkg/slackpkg.conf
+  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
+else
+  echo "==> slackpkg not found, assuming toolchain is already provided by the base image"
+fi
+
 # -----------------------------------------------------------------------------
 # 1. Baseline toolchain expected to already be present in the base image.
 # -----------------------------------------------------------------------------