fix: Add missing env var mention on upgrade help page

Add missing env defaults note for `--greedy` to `brew upgrade --help` output

Author: agowa

Library/Homebrew/cmd/upgrade.rb

@@ -91,6 +91,7 @@ class UpgradeCmd < AbstractCommand
           }],
           [:switch, "-g", "--greedy", {
             description: "Also include casks with `auto_updates true` or `version :latest`.",
+            env:         :upgrade_greedy,
           }],
           [:switch, "--greedy-latest", {
             description: "Also include casks with `version :latest`.",