CI: handle FreeBSD version of date(1)

ci/txcheck.sh

@@ -59,7 +59,7 @@ else
 fi
 # No unsaved changes; enforce a string freeze of one week
 DATE_PREV=$( git log -1 translation --date=unix | sed -e '/^Date:/s+.*:++p' -e d )
-DATE_HEAD=$( date +%s -d "1 week ago" )
+DATE_HEAD=$( date +%s -d "1 week ago" || date -v1w +%s )
 test "$DATE_PREV" -le "$DATE_HEAD" || { echo "! Translation tag has not aged enough." ; git log -1 translation ; exit 1 ; }
 
 # Tag is good, do real work of checking strings: collect names of relevant files