Improve markdown lint check (#15514)

Author: trask

.mise/tasks/lint/markdown.sh

@@ -1,10 +1,10 @@
 #!/usr/bin/env bash
 #MISE description="Lint markdown files"
-#MISE flag "--fix" help="Automatically fix issues"
+#USAGE flag "--fix" help="Automatically fix issues"
 
 set -e
 
-if [ "${usage_fix}" = "true" ]; then
+if [ "${usage_fix:-false}" = "true" ]; then
   markdownlint-cli2 --fix "**/*.md" "#**/build" "#CHANGELOG.md" "#licenses/licenses.md"
 else
   markdownlint-cli2 "**/*.md" "#**/build" "#CHANGELOG.md" "#licenses/licenses.md"