build: remove inconsistently used x-prefix comparisons Given that these are already used inconsistently with `test` checks, it makes sense to consolidate to not use the x-prefix comparisons at all. In any case, it would be nice to think we no longer have to work around a problem that existed in shells many, many years ago. More info: https://github.com/koalaman/shellcheck/wiki/SC2268 https://www.vidarholen.net/contents/blog/?p=1035