Fix frozen string bug

Library/Homebrew/exceptions.rb

@@ -466,7 +466,7 @@ class BuildFlagsError < RuntimeError
       require_text = "requires"
     end
 
-    message = <<~EOS
+    message = +<<~EOS
       The following #{flag_text}:
         #{flags.join(", ")}
       #{require_text} building tools, but none are installed.

Library/Homebrew/test/exceptions_spec.rb

@@ -225,3 +225,9 @@ describe BottleFormulaUnavailableError do
 
   its(:to_s) { is_expected.to match(/This bottle does not contain the formula file/) }
 end
+
+describe BuildFlagsError do
+  subject { described_class.new(["-s"]) }
+
+  its(:to_s) { is_expected.to match(/flag:\s+-s\nrequires building tools/) }
+end