audit: improve FileUtils check.

Match the parens properly and only interpolate once.

Closes Homebrew/homebrew#35288.
Closes Homebrew/homebrew#35291.

Library/Homebrew/cmd/audit.rb

@@ -548,7 +548,7 @@ class FormulaAuditor
       problem "#{$1} is unnecessary; just use #{$2}"
     end
 
-    if line =~ /system (["'](#{FILEUTILS_METHODS}))["' ]/
+    if line =~ /system (["'](#{FILEUTILS_METHODS})["' ])/o
       system = $1
       method = $2
       problem "Use the `#{method}` Ruby method instead of `system #{system}`"