ci: freeze luacheck version at 0.25.0

The newly released 0.26.0 emits a warning on the `_box` variable.
From luacheck v.0.26.0 release notes:

"Function arguments that start with a single underscore
get an "unused hint". Leaving them unused doesn't result
in a warning. Using them, on the other hand, is a
new warning (№ 214)."

NO_DOC=testing
NO_TEST=testing
NO_CHANGELOG=testing