fix(mise-tasks): handle unset CI variable in test script

Add missing check for unset CI environment variable in the is-in-ci function. The function now returns false immediately when CI is unset, preventing errors in the case statement when converting an unset variable to lowercase.

This ensures the script handles both unset and false CI values consistently without errors.

Author: liblaf

template/.config/copier/mise-tasks/test.sh

@@ -6,6 +6,9 @@ set -o nounset
 set -o pipefail
 
 function is-in-ci() {
+  if [[ -z ${CI:-} ]]; then
+    return 1
+  fi
   case "${CI,,}" in
     1 | on | true | y | yes) return 0 ;;
     0 | off | false | n | no | "") return 1 ;;