Fix fish shell integrations

I was confused about how the command substitution worked, in terms of parsing output. With bash and other shells, it'll split on whitespace, but in fish it only splits on newlines. As documented, use `string split -n " "` instead.

>Unlike other shells, fish does not split command substitutions on any whitespace (like spaces or tabs), only newlines. This can be an issue with commands like pkg-config that print what is meant to be multiple arguments on a single line. To split it on spaces too, use string split.

https://fishshell.com/docs/current/tutorial.html#command-substitutions
Signed-off-by: Daniel Mikusa <[email protected]>

src/scripts/fish.sh

@@ -1,7 +1,7 @@
 function docker;
     set DOCKER (which docker);
     if test "$argv[1]" = "run";
-        bt args -d | xargs -I{} $DOCKER run {} $argv[2..];
+        $DOCKER run (bt args -d | string split -n ' ') $argv[2..];
     else;
         $DOCKER $argv[1..];
     end;
@@ -10,7 +10,7 @@ end;
 function pack;
     set PACK (which pack);
     if test "$argv[1]" = "build";
-        bt args -p | xargs $PACK build $argv[2..];
+        $PACK build $argv[2..] (bt args -p | string split -n ' ');
     else;
         $PACK $argv[1..];
     end;