env_config: deprecate setting boolean vars to falsy values

The way we handle boolean environment variables is a bit unfortunate.
For example, setting `HOMEBREW_EVAL_ALL=false` actually enables
`HOMEBREW_EVAL_ALL`.

Let's fix this by deprecating setting boolean environment variables to
common false-y values (`false`, `0`, `nil`, `no`, and `off`) so that we
can later ignore these false-y values when reading boolean environment
variables.

Library/Homebrew/env_config.rb

@@ -515,7 +515,18 @@ def env_method_name(env, hash)
 
       if hash[:boolean]
         define_method(method_name) do
-          ENV[env].present?
+          env_value = ENV.fetch(env, nil)
+
+          falsy_values = %w[false no off nil 0]
+          if falsy_values.include(env_value&.downcase)
+            odeprecated "#{env}=#{env_value}", <<~EOS
+              If you wish to enable #{env}, #{env}=1
+              If you wish to disable #{env}, #{env}=
+            EOS
+          end
+
+          # TODO: Uncomment the remaining part of the line below after the deprecation/disable cycle.
+          env_value.present? # && !falsy_values.include(env_value.downcase)
         end
       elsif hash[:default].present?
         define_method(method_name) do