Fix syntax errors in Kissat checking script

In bash, `return` can only be used within a function. Use `exit`
instead.

scripts/check_kissat_version.sh

@@ -11,16 +11,16 @@ source "${KANI_DIR}/kani-dependencies"
 
 if [ -z "${KISSAT_VERSION:-}" ]; then
  echo "$0: ERROR: KISSAT_VERSION is not set"
- return 1
+ exit 1
 fi
 cmd="kissat --version"
 if kissat_version=$($cmd); then
  # Perform a lexicographic comparison of the version
  if [[ $kissat_version < $KISSAT_VERSION ]]; then
  echo "ERROR: Kissat version is $kissat_version. Expected at least $KISSAT_VERSION."
- return 1
+ exit 1
  fi
 else
  echo "ERROR: Couldn't run command '$cmd'"
- return 1
+ exit 1
 fi