fix: Use more idiomatic fish

The use of `.` is deprecated in fish, unlike POSIX shells. We can avoid
calling `psub` and pipe directly into source which does the same thing.

https://fishshell.com/docs/current/cmds/source.html

lib/asdf.fish

@@ -7,7 +7,7 @@ function asdf
     switch "$command"
         case "shell"
             # source commands that need to export variables
-            . (asdf export-shell-version fish $argv | psub)
+            command asdf export-shell-version fish $argv | source
         case '*'
             # forward other commands to asdf script
             command asdf "$command" $argv