services/pkg/mdbook: use mdBook-legacy to work around incompatibilities

Ref: void-linux/void-docs#840
Ref: void-linux/void-docs#841

Author: classabbyamp

services/pkg/mdbook/Dockerfile

@@ -1,6 +1,6 @@
 FROM ghcr.io/void-linux/void-glibc-full:latest
 
-RUN xbps-install -Suy xbps && xbps-install -y tini mdBook rsync git && \
+RUN xbps-install -Suy xbps && xbps-install -y tini mdBook-legacy rsync git && \
 	rm -rf /var/cache/xbps
 
 WORKDIR /mdbook

services/pkg/mdbook/build.sh

@@ -1,5 +1,5 @@
 #!/bin/sh
 set -eu
 git clone --filter=tree:0 "${REPO_URL}" /mdbook/repo
-mdbook build -d /mdbook/book/ /mdbook/repo/
+mdbook-legacy build -d /mdbook/book/ /mdbook/repo/
 rsync -a --delete /mdbook/book/html/* "${OUTDIR:-/out}"