fish: source each file in plugin conf.d separately

According to https://fishshell.com/docs/current/cmds/source.html,
only one file can be sourced at a time: "If additional arguments are
specified after the file name, they will be inserted into the $argv
variable."

PR #1204

modules/programs/fish.nix

@@ -441,7 +441,9 @@ in {
 
           # Source initialization code if it exists.
           if test -d $plugin_dir/conf.d
-            source $plugin_dir/conf.d/*.fish
+            for f in $plugin_dir/conf.d/*.fish
+              source $f
+            end
           end
 
           if test -f $plugin_dir/key_bindings.fish

tests/modules/programs/fish/plugins.nix

@@ -21,7 +21,9 @@ let
 
     # Source initialization code if it exists.
     if test -d $plugin_dir/conf.d
-      source $plugin_dir/conf.d/*.fish
+      for f in $plugin_dir/conf.d/*.fish
+        source $f
+      end
     end
 
     if test -f $plugin_dir/key_bindings.fish