Make from: required, fix flaky test

Library/Homebrew/cli_parser.rb

@@ -52,7 +52,7 @@ module Homebrew
         end
         process_option(*names, description)
         @parser.on(*names, *wrap_option_desc(description)) do
-          enable_switch(*names)
+          enable_switch(*names, from: :args)
         end
 
         names.each do |name|
@@ -173,7 +173,7 @@ module Homebrew
 
       private
 
-      def enable_switch(*names, from: :arg)
+      def enable_switch(*names, from:)
         names.each do |name|
           @switch_sources[option_to_name(name)] = from
           Homebrew.args["#{option_to_name(name)}?"] = true

Library/Homebrew/test/cli_parser_spec.rb

@@ -181,6 +181,7 @@ describe Homebrew::CLI::Parser do
     it "prioritizes cli arguments over env vars when they conflict" do
       allow(ENV).to receive(:[]).with("HOMEBREW_SWITCH_A").and_return("1")
       allow(ENV).to receive(:[]).with("HOMEBREW_SWITCH_B").and_return("0")
+      allow(ENV).to receive(:[])
       parser.parse(["--switch-b"])
       expect(Homebrew.args.switch_a?).to be_falsy
       expect(Homebrew.args.switch_b?).to be true