fix: Use builtin `source_env_if_exists` for sourcing `.envrc.local` (#6717)

.envrc

@@ -5,5 +5,4 @@ if has nix; then
   use flake
 fi
 
-# Source an `.envrc.local` if it exists. You can add your custom changes there.
-[[ -f .envrc.local ]] && source_env .envrc.local
+source_env_if_exists .envrc.local

.gitignore

@@ -5,6 +5,7 @@
 .DS_Store
 .vscode/
 .direnv/
+.envrc.local
 .flatpak-builder/
 zig-cache/
 .zig-cache/