starship: replace eval with source for fish

Changed the initialization method from `eval` to `source` as per the
starship installation guide.

See: https://starship.rs/guide/#%F0%9F%9A%80-installation

This is in accordance with the Fish docs which recommends using `source`
if the command does not need access to stdin, which in this case is what
upstream has.

See: https://fishshell.com/docs/current/cmds/eval.html

modules/programs/starship.nix

@@ -119,7 +119,7 @@ in {
 
     programs.fish.${initFish} = mkIf cfg.enableFishIntegration ''
       if test "$TERM" != "dumb"
-        eval (${starshipCmd} init fish)
+        ${starshipCmd} init fish | source
         ${lib.optionalString cfg.enableTransience "enable_transience"}
       end
     '';