determine-test-runners: test failure modes (again)

Library/Homebrew/extend/os/linux/dev-cmd/determine-test-runners.rb

@@ -197,6 +197,9 @@ module Homebrew
           dependents:       args.dependents?,
         )
 
+        github_run_id = ENV.fetch("GITHUB_RUN_ID") { raise "GITHUB_RUN_ID is not defined" }
+        github_run_attempt = ENV.fetch("GITHUB_RUN_ATTEMPT") { raise "GITHUB_RUN_ATTEMPT is not defined" }
+
         MacOSVersions::SYMBOLS.each_value do |version|
           macos_version = MacOS::Version.new(version)
           next if macos_version.outdated_release? || macos_version.prerelease?
@@ -211,7 +214,7 @@ module Homebrew
             next # No formulae to test on this macOS version.
           end
 
-          ephemeral_suffix = "-#{ENV.fetch("GITHUB_RUN_ID")}-#{ENV.fetch("GITHUB_RUN_ATTEMPT")}"
+          ephemeral_suffix = "-#{github_run_id}-#{github_run_attempt}"
           runners << { runner: "#{macos_version}#{ephemeral_suffix}", cleanup: false } if add_intel_runners
 
           next unless add_m1_runners

Library/Homebrew/test/dev-cmd/determine-test-runners_spec.rb

@@ -24,7 +24,7 @@ describe "brew determine-test-runners" do
       "HOMEBREW_LINUX_CLEANUP" => "false",
       "GITHUB_RUN_ID"          => ephemeral_suffix.split("-").second,
       "GITHUB_RUN_ATTEMPT"     => ephemeral_suffix.split("-").third,
-    }
+    }.freeze
   end
 
   it_behaves_like "parseable arguments"
@@ -35,6 +35,20 @@ describe "brew determine-test-runners" do
       .and be_a_failure
   end
 
+  it "fails when the necessary environment variables are missing", :integration_test, :needs_linux do
+    setup_test_formula "testball"
+
+    runner_env.each_key do |k|
+      runner_env_dup = runner_env.dup
+      runner_env_dup[k] = nil
+
+      expect { brew "determine-test-runners", "testball", runner_env_dup }
+        .to not_to_output.to_stdout
+        .and output("Error: #{k} is not defined\n").to_stderr
+        .and be_a_failure
+    end
+  end
+
   it "assigns all runners for formulae without any requirements", :integration_test, :needs_linux do
     setup_test_formula "testball"