Explicitly pass version-string to build, to avoid Git-based misdetection

Not sure if you already have another way of handling this, but current version seems to need this option in order for makepkg to work locally, otherwise it complains about the hardcoded app_version in build.zig "disagreeing" (with something which I'm guessing is derived wrongly from the src checkout somehow). Please let me know if this is a stupid misunderstanding, and thanks! 🙇