Make the Environment class's data members private

tests/gtest/avif_fuzztest_helpers.cc

diff --git a/tests/gtest/avif_fuzztest_helpers.cc b/tests/gtest/avif_fuzztest_helpers.cc
index 876008b..7d7c38c 100644
--- a/tests/gtest/avif_fuzztest_helpers.cc
+++ b/tests/gtest/avif_fuzztest_helpers.cc
@@ -203,6 +203,8 @@
   Environment(const char* name, const char* value)
       : name_(name), value_(value) {}
   void SetUp() override { setenv(name_, value_, 1); }
+
+ private:
   const char* name_;
   const char* value_;
 };