fix(fish): completions not working in 4.2.0+

[zappolowski]

Since fish 4.2.0 files in /usr/share/fish/ are ignored as they are embedded in the binary (they are still installed, though). Thus, all files therein are not loaded. /usr/share/fish/vendor_completions.d should be used instead as also indicated by pkgconf --variable=completionsdir fish.