Add resolver to workspace manifest

This explicitly specifies resolver "2" in the root manifest, to fix a
warning introduced in a recent nightly release.

Cargo.toml

@@ -5,6 +5,7 @@ members = [
     "alacritty_config",
     "alacritty_config_derive",
 ]
+resolver = "2"
 
 [profile.release]
 lto = "thin"