Fix CI: json-c for crun, and fetch crun's git submodules as pinned tarballs

Verified end-to-end with a full local build of all 8 packages on the
actual runner host (docker exec against vbatts/slackware:15.0, bind-
mounted repo on the cache pool) instead of round-tripping through
Gitea Actions for each fix — much faster for a chain of issues this
deep. All 8 packages now build successfully from a clean checkout.

Three fixes, all in crun (the last package still failing):

1. "Package requirements (json-c >= 0.14) were not met" — added
   json-c to the toolchain bootstrap.

2. crun depends on the libocispec git submodule, which in turn depends
   on the image-spec and runtime-spec git submodules. GitHub's source
   archive tarball never includes submodule content (no .git directory
   for `git submodule update` to work against — the existing `|| true`
   masked this failing silently). Fetched all three as their own
   pinned tarballs instead, at the exact commits crun 1.28 references
   (cross-checked via the GitHub contents API), matching how every
   other dependency in this project is already pinned.

3. crun.c unconditionally #includes git-version.h, which crun's own
   Makefile only generates via `git describe` (again, no .git) or from
   a pre-existing .tarball-git-version.h — the file crun's own `make
   dist` would have written, which we never run. Write that file
   ourselves in the exact format the Makefile already expects; this is
   the documented fallback path bundled release tarballs rely on, not
   a workaround around it.

Also includes the podman go-md2man pre-seed fix and setup-slackware-
buildenv.sh python3 addition from the previous commit's follow-up
testing (both already verified working in this same local build run).

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

scripts/ci/setup-slackware-buildenv.sh

@@ -78,7 +78,7 @@ if command -v slackpkg > /dev/null 2>&1; then
     gcc gcc-g++ binutils make m4 perl autoconf automake libtool pkg-config \
     curl nghttp2 brotli cyrus-sasl ca-certificates glib2 libcap fuse3 \
     cmake libarchive lz4 libxml2 guile gc kernel-headers flex elfutils \
-    python3
+    python3 json-c
 else
   echo "==> slackpkg not found, assuming toolchain is already provided by the base image"
 fi
@@ -104,6 +104,7 @@ require_binary python3 "Needed by crun's configure script (checks for Python >=
 require_pkgconfig glib-2.0 "Install Slackware's glib2 package (needed by conmon)."
 require_pkgconfig libcap "Install Slackware's libcap package (needed by crun)." || true
 require_pkgconfig fuse3 "Install Slackware's fuse3 package (needed by fuse-overlayfs)." || true
+require_pkgconfig json-c "Install Slackware's json-c package (needed by crun, >= 0.14)." || true
 
 # -----------------------------------------------------------------------------
 # 3. libseccomp — not part of stock Slackware, build from source if missing.