[test] avoid leaking temp variable

Author: ryenus

test/fast/Unit tests/nvm_print_versions

@@ -8,6 +8,7 @@ cleanup() {
   unset -f nvm_remote_versions nvm_ls nvm_ls_current
   if [ -n "$TEMP_NVM_MIN" ]; then
     export NVM_MIN="$TEMP_NVM_MIN"
+    unset TEMP_NVM_MIN
   fi
 }