Fix RuboCop: Use ENV.fetch instead of ENV[] for compiler flags

Author: sbryngelson

packaging/homebrew/mfc.rb

@@ -58,9 +58,9 @@ def install
       system venv/"bin/python", "-m", "SCons", "build",
              "CC=#{ENV.cc}",
              "CXX=#{ENV.cxx}",
-             "CXXFLAGS=#{ENV["CXXFLAGS"]}",
-             "CCFLAGS=#{ENV["CFLAGS"]}",
-             "CPPFLAGS=#{ENV["CPPFLAGS"]}",
+             "CXXFLAGS=#{ENV.fetch("CXXFLAGS", nil)}",
+             "CCFLAGS=#{ENV.fetch("CFLAGS", nil)}",
+             "CPPFLAGS=#{ENV.fetch("CPPFLAGS", nil)}",
              "python_package=y",
              "f90_interface=n",
              "system_sundials=y",