CI: install mandown for validate

Add target to install mandown, also fixes useless `command -v` exucuting
in the makefile since it did't error out anyway when it was empty.

Signed-off-by: Paul Holzinger <pholzing@redhat.com>

contrib/cirrus/runner.sh

@@ -39,6 +39,7 @@ _run_build_aarch64() {
 }
 
 _run_validate() {
+    make -C docs .install.mandown
     make validate
 }
 

docs/Makefile

@@ -1,13 +1,17 @@
 PREFIX ?= /usr/local
 DATADIR ?= ${PREFIX}/share
 MANDIR ?= $(DATADIR)/man
-MANDOWN ?= $(shell export PATH; command -v mandown)
+MANDOWN ?= mandown
 
 docs: $(patsubst %.md,%,$(wildcard *.1.md))
 
 %.1: %.1.md
 	$(MANDOWN) $< > $@
 
+.PHONY: .install.mandown
+.install.mandown:
+	cargo install mandown
+
 .PHONY: install
 install:
 	install -d ${DESTDIR}/${MANDIR}/man1