audit: tighten xcodebuild SYMROOT check

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Homebrew/cmd/audit.rb

@@ -102,7 +102,7 @@ def audit_formula_text name, text
   end unless name == "gfortran" # Gfortran itself has this text in the caveats
 
   # xcodebuild should specify SYMROOT
-  if text =~ /xcodebuild/ and not text =~ /SYMROOT=/
+  if text =~ /system\s+['"]xcodebuild/ and not text =~ /SYMROOT=/
     problems << " * xcodebuild should be passed an explicit \"SYMROOT\""
   end